To write or not to write gramatically correct

Why write grammatically correct? It's a impositive question for me, because, as I read in The Structure of Magic (a book about language and therapy), natives can speak correct because they know intuitivelly the structure of their idiom. But the strutucture of a idioms it's what only matters?

I doubt that question because there is much more to be discovered in a idiom, grammatically speaking who people can understand, but we don't need to have that issues when talking with cultured people.

So, there are grammatical discoveries too.