Is this sentence supposed to be a rhetorical question through and through, emphasising the uselessness of using the correct grammar?
= "What difference does it make whether you learn the grammar or not?" {ironically}
On the other hand, if you are really interested to find out about the difference/benefits it makes by learning the grammar, how would you rephrase the sentence? Is it possible to start the sentence with « Quelle différence ... », or would you rather say something like:
Que gagne-t-on à apprendre la grammaire ?
I suppose that it is easy for English speakers to fall into the trap of using the sentence in the title, with a non-sarcastic real question in mind.