Sometimes it's hard to detect sarcasm on here, lol, but if you're seriously asking, it's because of play styles. Sometimes running your main can grow old. Switching over to a different character changes some of the ways you play so it keeps the game interesting when it comes to engagements. Also some supers are more fun to play with than others. It used to be really easy to run 3 characters time wise in D1. As D2 has grown there's so much to do on my main that my alts catch dust most of the time.