Yes, if you and I are talking to each other and understand each other, it doesn't matter whether we're using "proper English" or not.(posted 8751 days ago)But if you are writing an article that I and hundreds of other people are going to read, or if we're co-workers who have never met and you have to send me a message about something important, then we shouldn't have to waste time establishing a common understanding of terms and grammar before getting to the content of your message. We need to have a standard that all of us know about in advance. So if you're going to be communicating in the English-speaking academic and business world, you should know and use the standard forms of English used in that world.
Yes, these rules are arbitrary; the definition of "kilogram" is also pretty arbitrary, but it's a good thing we all agree on how much a kilogram weighs. Yes, knowing these rules is trivial compared with knowing, say, advanced economic theory; that's why people should know them before they graduate from high school. The people who teach writing to graduate students are making a last-ditch attempt to make up for failure in the rest of the educational system.
In my work as a technical writer, I have to read the white papers that programmers write to summarize new features of my company's products. When smart people who can't write have to communicate in print, the results aren't pretty.