I have four chars that I play. I have a favorite, but they all bring different things to the table. The playing style required for dif chars is an example. As a paladin or Mage, my playing requires constant monitoring of other's health, positions, etc. Playing a bear or an archer build allows me to develop other skills. I also learn more about how each char plays within a team if I play them myself. Also, if there aren't new levels out, it's something new to do...

But, above all else, different characters are fun