Ever wondered why your English teacher told you that you couldn't split an infinitive (to go, to run, to kill etc)? It's because Latin (which English did not come from, more on that later) does not have split infinitives. Why? Because Latin infinitives are only one word!
She wants to quickly run to the store.
She wants to run to the store quickly.
Both of the above sentences make sense. They are equally 'correct'.
Your teacher probably also told you to never start a sentence with a conjunction. And I just did, and you knew exactly what I meant. My proverbial tongue is sticking out.
What about that pesky contraction "ain't"? You might have been raised hearing the expression, "Ain't ain't a word!" but it totally is. In fact, 'ain't' is old as dirt and twice as common. If native speakers use it and understand it, it's a word folks.
We as linguists want to make sense of the way people actually use language, not attempt to warp people's usage to suit our fancy. We do fieldwork, we withhold judgement on 'slang' and 'colloquialisms', and we have confidence that languages work without our help. Instead of controlling them, we want to know what makes seemingly unregulated chaos serve us so efficiently as speakers.
There are some uses to the perspective approach but, more often than not, the prescriptive approach to language (think grammar nazis) is complete CRAP.
So in summary, linguists describe how language is used. We do not prescribe how it should be used.