Throughout history, language has always changed. Way back in Anglo-Saxon times, there weren't really any rules, and regional differences in spelling and grammar were common. However, as the English language spread across the world and literacy finally expanded beyond the clerical and upper classes, it became increasingly important to standardize at least somewhat.
Enter Doctor Johnson and his dictionary, which finally popularized the idea of a "wrong" spelling or "bad" grammar. From there, it was a short hop and a skip to elementary schools across the world insisting that kids learn how best to structure their sentences.
At its best, grammar is not about excluding anyone on the grounds of their education, and it is certainly not about making anyone feel stupid. It's about conveying meaning in a way that is clear, thoughtful, and accessible. Good grammar is about using the right words in the right way to get an idea across. It varies between cultures and even subcultures, meaning that the best grammarian will know how to code switch rather than slavishly sticking to whatever is considered "proper."
Proper grammar means speaking to the intended audience on their level and giving your thoughts the best chance at truly being heard and understood. It is elegant and inclusive, willing to change with time or yield before common usage. Indeed, the best grammar often lies in knowing exactly when to break the rules, for example, Star Trek's famous split infinitive, "To boldly go where no man has gone before." As Churchill said, speaking to a grammar prude, "This the sort of pettifogging nitpicking up with which I will not put!" So let's make sure you know all the rules because then you'll know when to use them - and when to break the heck out of them!