It’s one of those aspects of the Society of Discipline that enclosures of experts form to put knowledge into hierarchical strata and assess who gets power and who does not based on the rules of the enclosure. Yes, I’m talking to you MLA and OED.
It is indeed important to learn the accepted linguistic conventions of the standard dialect for reasons of communication, clarity and even persuasive style. But it happens to be a historically privileged dialect through no innate merit of its own and is not inherently linguistically better than other, non-standard dialects of English. Even if you don’t buy this linguistic fact, like all dialects, even ones you may perceive to be ungrammatical, there are rules which reflect how speakers actually use the language. These rules are not formed by some invisible authority on high, never to be questioned, ever.