Despite apparent opposition, prescription and description can inform each other,[page needed] because comprehensive descriptive accounts must take into account speaker attitudes (including prescriptive ones), and some understanding of how language is actually used is necessary for prescription to be effective.
But I've just never really understood prescriptive grammar -- it seems like language is constantly being added to (and to a lesser extent, losing some words because they're becoming so archaic it's not even really the same language anymore), so saying what's "right" or "wrong" in terms of grammar is basically choosing some specific moment in time and arbitrarily deciding that that's the "proper" form of the language, and the one to strive for.
It just seems like an inherently losing battle, and one that makes no sense to fight -- if 100% of people understand a word, what's the point in saying it shouldn't be used? E.g., "I ain't" is definitely not considered correct, but there's no real reason it shouldn't be allowed as an alternative to "I'm not".
I understand one reason for prescriptive grammar, essentially to make communication possible. It essentially establishes rules, and speech would quickly get confusing if we had no rules. But some of the things it prohibits seem to have little to do with facilitating communication, and more with just upholding some arbitrary status quo. "Swag" is a really annoying word, but it doesn't really make sense to say it's not an "official word" or something if millions of people are using it and all know what it means among each other.
Why exactly do prescriptivists even really want this? Is it some sort of weak attempt at "seeking permanence in an impermanent world"?