First of all, it's important to acknowledge that in a perfect world no one would be judged based on anything other than their actions and true personality. However, the reality is that we all make split-second decisions about each other based on all sorts of less reliable factors. Language is one of those factors, and it's not too hard to figure out why.
Think about a time when you've met someone for the first time. Before you had a chance to really get to know the person, your mind had probably already started to form a first impression based on the way the person looks, speaks, and acts. When you know you're going to be meeting a lot of people for the first time, you may find yourself dressing a certain way or trying to use your very best manners so that you'll make a good first impression, too.
When someone speaks with proper grammar and displays a large vocabulary, we tend to assume that the person is well-educated and trustworthy. When someone uses a lot of slang and incorrect grammar, you may have less confidence in what he or she is saying (at least initially). Although these generalizations may not turn out to be accurate in the long run, people usually want to present the best possible image of themselves by using grammatically correct language in formal situations. (When you're with your family and friends, it's okay to relax.)
As you think more about the ways in which language influences first impressions, you might come up with different types of situations where perhaps more trendy (less formal) language would actually be the most beneficial. The best-case scenario is to be well-versed in several modes of language so that you can choose your words to fit the occasion and communicate well with everyone.