There is an abundance of social stratification in language, and the ability to code switch in different social environments is an important skill. It doesn't matter that you would still be understood if you did not do so, what matters are the pragmatics and social labeling attached to speaking in/out of the current group's mode of speaking.
It is true that the formal study of linguistics (which was my own area of advanced degree) does not consider there to be an absolute correct way of speaking, that "grammar" is not some unchanging and inherently correct set of rules. They are, instead, a social convention. And like many other social conventions, breaking their rules has consequences, and understanding those rules and consequences is important to navigating in social environments.