People hate having their grammar corrected. And when you do correct somebody's grammar, the #1 response is, "I don't care what's right. You understood what I was saying." And, you know what? It's true. I do understand what people are saying, or trying to say. I can figure out typos, missing punctuation, incorrect word usage, and all the countless errors I come across in written language. But I shouldn't have to.
Standard written English is a level playing field. It is a common language that people of all dialects and classes can understand. The more we see it, the deeper it is ingrained into our minds. We can read and understand very quickly because we have guidelines that streamline the process. When we deviate from the standards, it slows down and complicates the process.
The more errors there are in your writing, the longer it takes me to recognize an error, figure out what caused the error, what the likely intention was, and processing that information. One missed keystroke here and there can make for a bumpy ride. But several mistakes in each sentence will turn that ride into a crawl.
When you use proper grammar, you are being courteous to your readers. You are saving them time and trouble from having to figure out what you meant to say, because you actually said what you meant. On top of that, you are also helping yourself by making people happier and more willing to read your work, because of how simple the process is. There's really no reason not to do it (except for being lazy).