What does it mean to communicate effectively?

In my opinion, communicating effectively is to get your point across however you can, whether you're using "correct grammar" or not. In teachers' opinions it seems to be using the correct word choice, punctuation, tense, and even word order. What I don't understand is if they can get what you are saying, why does it matter? Looking at grammar help websites they talk about interjections, determiners, pronoun consistency, primer language, and many more foreign terms. If these are so important to teachers why do we not constantly go over them instead of reading books? 

I know I'm biased in this blog because I am terrible at grammar. When I was younger my mom had to buy me a computer grammar game so I would learn not to make certain words plural and other grammar nonsense. Well my question here is why did my mom have to go out and buy me this game in order to learn it? Why wasn't I learning it in school? I think sometimes the case was that I didn't learn as quickly so I probably fell behind but I also don't remember my teachers really explaining why you should write a certain way. Whenever I got a paper back it just had the correction without an explanation so I wasn't learning the grammar rules, I was just replacing or changing a word for no reason. 

Now don't get me wrong, I do not think all grammar is useless. I think it is necessary to have some rules to writing because, like someone in class said, it creates a common ground for anyone learning the language. It also teaches you how to write and this is important in an academic setting where you may be giving a speech, or you may be writing for a job. My point is more of on the lines of when grammar gets too nit picky. I think it has gotten way out of hand that teachers expect students to know certain "rules" that students were never taught. Like I said before, if they are going to take off that many points from your paper then they should spend some time going over their rules. I do feel uneducated in a sense when I don't know grammar and people correct me but the point shouldn't be how we word something, it should just be about comprehending what one is communicating.