You didn't think I'd get through a whole lesson without mentioning spelling, grammar, and punctuation, did you? In this college composition course, you learn a lot about what makes a sentence right or wrong grammatically and punctuation-wise, and those specifics are important, but they're not important in and of themselves. They're important because every mistake you make weakens your writing a little bit more. Why? Because mistakes are distracting! Worse, if you're trying to make a persuasive argument, those mistakes are going to ruin your credibility.