Why did Obi-Wan have to explain full details about Mos Eisley to Luke?

There’s definitely exposition for the audience, but even in-world Obi-Wan is much more experienced than Luke is. Luke may know the place is a rough area, but “old Ben” who fought in the Clone Wars is basically telling him that it’s one of the toughest places he’s seen. It’s a warning on top of whatever rumors Luke may have heard.

Or... it’s just commentary. People say unnecessary things all the time. It’s kind of human nature

I'd like to offer an in-universe answer.

Owen Lars, Luke's uncle, is a very conservative person. Living on a relatively outlaw world with species aggressive towards humans such as the Sand People, he is focussed on his farm.

He's been given responsibility for Luke, almost certainly knowing his true parentage, he tries to bring him up as a farmer and tries to discourage his tendency to want to leave. At the start of the movie there is an exchange about him going to the Academy, which Owen refuses:

OWEN You must understand I need you here, Luke.

LUKE But it's a whole 'nother year.

OWEN Look, it's only one more season.

LUKE Yeah, that's what you said last year when Biggs and Tank left.

Some of this fear of the unknown and conservative nature has rubbed off on Luke. When Ben/Obi-wan suggests that Luke accompany him taking the plans to Alderaan, he is suddenly nervous:

LUKE I can't get involved! I've got work to do! It's not that I like the Empire. I hate it! But there's nothing I can do about it right now. It's such a long way from here.

BEN That's your uncle talking.

LUKE Oh boy, my uncle. How am I ever going to explain this?

BEN Learn about the Force, Luke.

LUKE Look, I can take you as far as Anchorhead. You can get a transport there to Mos Eisley or wherever you're going.

Anchorhead is the nearest small town to the farm, but given that Mos Eisley is a spaceport and Owen is nervous about Luke leaving it seems very likely that Luke has never been allowed to travel as far as Mos Eisley.

So when Ben and Luke stop and we see them overlooking the city, it's very likely that this is the first time Luke has been there. Ben isn't explaining that Mos Eisley is a space port, or where it is. It's clear that Luke already knew that it was the nearest place to get off-planet.

Everything we've seen about Luke so far is that he has lived a very sheltered life, a product of his upbringing on a remote farm by an arguably overprotective uncle. Ben's line is just a comment, giving Luke a warning that this place is rougher than he is used to.

It's called exposition, an info-dump or idiot lecture.

A plot device using characters to explain to each other what the audience needs to know about a place, character, event, history, or other or background information necessary to the plot, hopefully without the audience noticing this is being done entirely for their benefit.

Exposition is the most subtle of these and can be done by a skilful writer without the audience being aware it's happening.

At the other end of the scale is the 'idiot lecture' where characters are discussing things necessary for the audience to know, but which the characters in-universe ought to already be perfectly aware of.

As the quote has now been edited in, I'd say that's 'simple exposition' - one character telling another something he didn't already know, which we, the audience, also need to know.

Mom is taking her teenage son to the mall to buy some new pants. They get there and mom says, "Remember Jeff: we've only got an hour before we need to head over to the book store."

Jeff replies, "Yeah, mom, I know, you've told me a hundred times."

It's theoretically possible that it's the "I know we both know this but I'll feel better if I say it out loud" concept. Obi-wan knows it's a dump; Luke knows it's a dump. They both know that they need to be careful. Obi-wan just can't set his mind at ease until he voices what's on his mind.

Earlier in the movie Luke went out into the wastes to find R2-D2 and was attacked by sand people and saved by Obi-Wan.

Also against Obi-Wan's advice he return to his home that had been just been attacked by Storm Troopers.

So perhaps Obi-Wan might think that Luke is young kid that would run blindly into danger, and could use a reminder to try avoid getting himself killed in Mos Eisley. The reminder didn't seem to work too well since Obi-Wan had to save him from Ponda Baba.

It's a re-affirmation of the archetypal journey to the underground where the boy will have a chance to become the hero.

It's part of his character as an upstanding person. Luke's Uncle Owen called Ben a "Crazy Old Hermit". Hey, maybe on Tatooine, crazy old hermits head to Mos Eisley on the weekend to partake in debauchery.

But not old Ben Kenobi-- he's passing judgement on this place, and he doesn't approve of it.

As I know from the movie, Luke has never been to Mos Eisely before. Because Luke's uncle Owen raised him like that. Luke had only been as far as Anchorhead, so Obi-Wan has to explain the details about the place Mos Eisley.