If you assume God's omnipotence/omniscience to be a given, why does he communicate with anyone at all?

Why does he use his voice, or even English? Why does he need a physical representation?

Also, why do we (adults) use simple language when we talk to a child, when we ourselves are smarter than that?

Because it's what the other person is capable of dealing with.

If God could do everything by himself, he wouldn't need to communicate anything with anyone. The fact that he communicates anything (or even just confirms his existence) means there is a purpose to it. He needs them to understand, and is therefore explaining it (and himself) in a way that is easily understandable. Demons use the human form to connect with humans, why shouldn't he?

Asking where the shower is is just a polite way of asking to use the shower. It doesn't have to imply that he doesn't know. I know where the coffee maker is in my friend's apartment, but I still ask him if I can go get a cup of coffee.

Edit: If anything, the quote actually shows that he knows the location of the shower.

Where's the guest room? I could use a shower.

He already knows it's in the guest room (and that there is a guest room). Why doesn't he ask for the bathroom?

God is The Trickster's (archangel Gabriel's) father ! ...

Many characters/scripts are developed in the series in such a way as to show how one matter or main character affects another.

(Maybe needless example. When we see the interaction between God/Michael and Lucifer with their respective vessels/swords in the early show it intends to show how important Sam and Dean can get to be and how they are considered in the big picture. So S & D are important because of their past, how they were raised, and so on. Likewise, some personality traits are present too when Metatron explains his, many times passive, role through all times. However he chooses something different when needed.)

God being Trickster's father may convey the idea that all his creations have some relationship to his own personality. Through season 11 He shows how He enjoys doing several human like activities such as eating bacon and watching tv, these characteristics being pretty much alike to that of Gabriel's. That could even consider types of porn, since Gabriel also was a bit of a fan.

Of course politeness (good point on another posted answer) can play a role in the God-Human interactions, but this alone may not be enough.

As an end note: perhaps He has some pleasure in checking people's reactions, just like the Trickster. He could just as well say:

• Hey guys I feel like taking a shower are you all cool?

Same thing but with no need to ask for the place where it is. As the conversation actually went, it could have been answered with some slight giggles, and

• Sure you can go, you know where it is.

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Also, on the following episode, God says that He has some guilty pleasures. Although not sure if this may, or may not, be enough to explain why he watched xxx ...