My personal take is that the vulgarity serves two purposes - directed outword it condemns deliberate ignorance, unwillingness to reconsider entrenched beliefs, and often just outright monstrous beliefs as unworthy of further discussion. It's also often deployed after someone brings out slurs.

Essentially, it's a vulgar but harmless response used when a discussion has strayed beyond the point where good faith can be extended.

Internally, i think it's part of the anti-civility culture of dirtbag leftists. Vulgarity rejects civility discourse and respectability politics. I think it's notable that our transgressive language, while gross, doesn't target any group of people or involve prejudice or oppressive language. It's offensive to people who value "civil discussion", but ultimately harmless. In contrast with other online cultures who try to be "edgy" using slurs and bigotted language.

I guess a third function is that it's silly and absurd, and serves to bring the mood up. We spend a lot of time talking about absolutely horrific crimes against humanity and unspeakable violence. The prospects for the future are not good. The ridiculous peepee poopoo language breaks that up, sort of shocks the mind out of doomerism and despair. For me at least it's good for morale.