In general, I suspect that it emboldens people and means that aspects of their own personality (that they usually keep in check for real life interactions) will tend to come out. Sure, there are people who are "the same online as they are IRL", but even then, there tends to be an amplification of some/all aspects of their personality. Additionally, anonymity provides an opportunity (for those who are interested or curious to try) to "be someone else" and explore what happens when they act completely unlike their real self. But in order for them to "test run" an alternate personality, that persona has to come from somewhere, and unless they are parroting someone else, it's come from within themselves.