Grammar is something we study and come to understand. "Proper" grammar are guidelines used to communicate to as many people as possible, easily interpreted (as a text can get) and easily accessible. "Proper" grammar, which really should be called "good" grammar, although even that may be a bit much, involves writing clearly, concisely and logically.
The reason we teach grammar is that the students, despite speaking English, are speaking foreign languages to each other. Deciding on a "proper" grammar gives a foothold to a standardized language that we (should) all have access to, so that miscommunication can be avoided as much as possible.
One could argue a credibility issue. If a person uses semi-colons where a comma should be placed, every time, and if that person writes a good amount, say for research or a business, then what does it say about that person? To write so much and not be familiar as to the difference between a semi-colon and comma? Not only would it be a basic observation (if you write a lot, you do read a good deal, enough to come across the difference) but it would call into question the person's research skills.
Whether or not the above is my argument is not something I care to consider right now. But it is an illustration, one that could be fleshed out better.
Considering grammar and science come from the same parent, shouldn’t there be an appreciation on the teacher’s part for grammar? One could say that the movement to grade twice a year is proof of an appreciation; but if that were true, why utter such a question as “Why do we need to teach grammar?”
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