Grammar matters because without it, a lot of sentences would be ambiguous. For example: "He I give advice". In that sentence, who is giving advice and who is receiving it. And when - now, yesterday, tomrrow? One could argue that there is too much grammar in some languages, but you will find most rules which are regarded as redundnat come in useful sometime.

"Why does grammar matter?" hmm, let me put it it this way... Does why matter grammar? Why grammar matter is? Why does grammars matters? Why is do grammar to matter? Matter grammar's does whys? It's pretty clear, don't you think? ;)

What exactly is the point of this question?