On the one hand, it’s a group of arbitrary rules established by a bunch of old dead men who wanted to preserve some shadow of Latin constructs in modern language, and said rules have become stifling and largely unnecessary for thorough and comprehensible communication.
But on the other hand, it’s become something more than a set of rules—it’s become a social norm. Prescriptivist rules are so ingrained into our collective minds that when someone chooses not to follow them, they are, unfortunately, often perceived as less educated, regardless of content or language mastery.
Grammar is a bit like business formal: when you become more established, you have more opportunities to flaunt your own style. But for a job interview, you follow the rules and wear whatever is going to get you hired.
It still is, unfortunately, a discriminatory practice, especially when it comes to non-native speakers. But it seems as though we’re going to be stuck with it for a while still.