Science

I agree with most of what you said so I'll focus only on a specific point, OK?

That’s the point of language. Convey information. If the information is conveyed, then language has done its job. Yay language.

There's another point of language, besides conveying information, that is relevant here: identity. Different people speak different varieties because this allows them to express "this is who I am, I speak like my peers".

That's why AAE speakers use their varieties on first place - it's a way to tell the world "I'm black, this is who I am, I identify myself with other black people". And also why those varieties get such a stigma - because in USA, they want people to feel bad for being who they are, if they're black. (NB: I'm not from USA, but this is so fucking obvious that even an external observer gets it.)