Why does Quentin Tarantino add non-plot related dialogue in his movies?

Not everything has to drive the plot.

His characters are supposed to be real people. Real people have those types of conversations.

It humanizes the characters, whilst, at the same time, setting up the disconnect between their words and their actions.

Take that hamburger conversation..

They're on their way to work, not at work. So they catch up, talk about their lives and other minutae not related to work, just like ordinary folks. When they get to work... they switch modes.

But even then, they were having a conversation about burgers which ties into the subsequent action when they recover that briefcase.

Here's an excellent comment I found which explains it perfectly.

In Pulp Fiction, almost every line is brilliant in a distinct way. From the poetic prose of some of the monologues, to the quick and profane exchanges between actors, this movie has it all, and it is all great. Tarantino writes with a knack for realistic dialogue. His characters talk about real things and often discuss day-to-day proceedings which seem irrelevant to the plot. This brings the characters to life, but what QT also often does is subtly slip that seemingly random dialogue back into the plot.

For example, in Pulp Fiction Jules (Samuel L. Jackson) and Vincent (John Travolta) have the best written conversation about hamburgers in the history of cinema. They compare quarter pounders in America to those in France while on the way to kill some young men who ripped off their boss. The French name for the burger reappears in the dialogue later when they’re “in character” in the apartment of the guys they’ve been sent to whack. The use of such ordinary conversation is almost hilarious at times, as Tarantino’s characters may be discussing the simplest things while carrying out a contrasting action- like stashing a body or dealing drugs.

^Source

Because the plot is not the narrative

A simple reading of a film plot will often show the primary narrative structure of a film: greed is harmful, and so the greedy villain is punished by losing their property, protagonists fighting violence with violence leads to the death of their loved ones etc. But there are many other ways to build up a narrative and to express meaning outside the plot: using visual artistry, music or dialogue. This allows a film-maker like QT to express more than just through action and plot dialogue alone.

For instance in Pulp Fiction, many of the dialogues (while plot-unrelated) serve both to familiarize and introduce the characters, but also set up narrative themes such as the banality of evil. Comparisons of food as well as crime and drug laws in the US and Europe show that the criminal life of the characters is for them a thing of choice and pride, and not how things need to be. It may seem plot-unrelated, but the discussion of eating pork has many direct implications on the narrative: what do the characters consider clean, what is unethical, how little do they value human life compared to that of a dog or a pig?

Another significant reason for unrelated plot dialogue in QT's work is pacing and tension. For instance in Hateful 8, while many of the conversations are narrative-building (discussing the history and race relations after and during the US civil war), they are also used to express the slow passing of time, to create mental breaks between moments of action/tension and as a backdrop to what is happening visually. Allowing viewers to relax between action-packed scenes can both help emphasize the action as well as not overwhelm them with too many things going on at once.

So to sum up I'd say that instead of wondering why QT's films contain non-plot dialog it's better to view the plot of a film as a single contributor to the overall narrative experience, alongside (plot and non-plot) dialogue, visuals, audio and other narrative devices.

It makes the characters more real and relate-able. They seem more like actual people than caricatures playing rubber-stamped movie prototype roles.

Actual people, when interacting, most of the time, don't have matters of grave import to discuss. So they have discussions that are goofy, trivial, insipid and mundane. Also, the way the characters try to shoe-horn some of these trivial discussions into a larger philosophy is a way of showing the character beliefs and giving us insight into how they think without having them make a pontificating speech that serves no purpose other than to drive or support the plot of a movie (important to a movie, but not really a consideration for a real-life individual) - again, making it more like a real-life interaction.

Plus Quentin thinks he's pretty clever and witty, and this is his way of showing that off.

One simple, superficial answer is that Tarantino is a genius at juxtaposing violence and comedy. Just when the violence gets too much, he flips to something absurd.

However there is more to it than that, and the premise of your question seems to me to be entirely incorrect. Each of the scenes you mention does not seem to relate to the plot when you watch it, but they set up another scene later in the movie.

In Pulp Fiction:

The Royale with cheese is perhaps the best example of "non plot related dialogue" you mention. It is a very inconsequential subject. But Jools does randomly bring it up again,

in the middle of intimidating someone, which helps to show what a nonchalant, cold blooded character he is.

The foot massage is clearly related to the plot. Vince argues that his boss Marcellus was justifiably angered that someone gave his wife a foot massage,

but is later attracted to Marcellus's wife, and ends up nearly killing her due to misadventure with drugs.

In Reservoir dogs:

The other robbers are angered that Mr Pink refuses to do the honourable thing and tip the waitress, but he does not back down because he will not be swayed by societal norms. This is in keeping with his character later,

Because he forms no allegiances except to himself, and thus avoids the shootout and ends up with the loot.

And finally another example I'd like to throw in, in Inglourious Basterds (not dialogue but the same principle applies):

Aldo carves a swastika on a Nazi soldier's forehead, apparently of little significance to the plot

but later when he prepares to punish another, more major character, we know exactly what he is going to do.

Tarantino is not the only writer/director to do this (most of them do!) but he is good at it.

I believe its a technique he likes to use to heighten the impact of the violent scenes in his movies. If you were accustomed to every moment being an action movie scene, those action sequences wouldn't quite have the same punch.

In the scenes you mentioned are perhaps a more general use of the technique, but I've observed at least two situations where Tarintino used particularly banal banter to lull the viewer into mental torpor right before a big shock of violence. This (in me at least) drastically increased the shock of the violence when it happened.

The first example is also in Pulp Fiction, where three characters are having a conversation about the supernatural, when a gun misfires and kills one of them. ("Oh man, I shot Marvin in the face"). The other example is the bar shootout scene in Inglorious Bastards. It was about 25 minutes of (IMHO dull) dialog in German with subtitles right before all hell broke lose.

For the Reservoir Dogs example, it's a smaller story before the larger story and in my opinion drives a underlying question. "Is there honor among thieves?"

The bit about Madonna's "Like a virgin" is meant to establish time and place, as well as culture.

The argument about tipping should appear almost humorous as a small 'honor among thieves' bit. Even crooks should do right by those less fortunate. Right?

In fact we can pour pretty heavily into ethics in this movie.

SPOILERS BELOW

Mr. Pink is a professional: he's in it for himself and he's not a rat. He is pretty neutral, neither dishonorable nor honorable as a thief.

And

Mr. Orange is the undercover cop. He spent a long time sneaking into this circle. He's betrayed the lot. This actually makes him pretty evil/dishonorable (even considering this is evil for good's sake).

And

Mr. Blonde is clearly a sociopathic thief, and very evil, very dishonorable.

And

Mr. White's tragic mistake is falling for Mr. Orange's cover. He believes so truly that Mr. Orange is a "good" robber. He makes an ethical decision to stand by Mr. Orange.

And

The horror is revealed in the last scene, by Mr. Orange, why?

And

The Motive in my opinion is honor.

And

Mr. Orange knows he is about to die and won't make it, and feels compelled by guilt to tell Mr. White the truth, as horrific as it is.

And

Because Mr. Orange watched Mr. White gun down his friend/boss defending Mr. Orange, a "dishonorable thief".

These scenes don't need to drive the plot. Simply, they don't have too. Often they help establish a few very small things with the setting and characters. It's world building.

But in Reservoir Dogs in particular there is a lot going on in this scene.