Grammar existed long before anyone named it or ever described any rule of grammar. It's something people do instinctively: develop systems, for using our word-symbols to get ideas across. Sign language, for instance, was assumed - even by its speakers - to be just a list of sign-words, until some bright researchers pointed out the spacial grammar and inflections of direction, speed, size, facial expression, etc. which people had developed and were consistently using without consciously thinking about it. Another natural language had evolved under our noses.

The rules of grammar are not there to tell us what to do, they are descriptions of how people use language. Prescriptive pedantry can give the word "grammar" a bad name, but we simply cannot get our meaning across in speech without following some kind of grammar. The alternative is babbling: "mean one what something," for example.