I personally like creating alts for several reasons, the main one being the increasing boredom that overcomes a 70 toon. That can happen due to the daily routines, be it RP , PvE or PvP , it comes a time where you realize you are repeating stuff or actually doing nothing at all and you frown. That's the time where creating an alt is crucial for you to:
a) keep yourself entertained
b) Get some resting time from your main char and eventually start to miss playing with it and comeback with new ideas
c) get to know other classes or the other faction
For some time I was the guy who thought "Who needs alts?" but when I started to get
bored from the daily routines and the lack of fresh ideas, it was creating alts that kept me amused in WoW,