There are a million reasons why someone with a high main would start an alt. However, I wondered if there was a thread needed to share the practical reasons (as opposed to boredom) that people have started little alts. My three reasons...
1) A little mule as a trader.. it saves all the messy bank space problems with loaded trader bags.
2) A nice ranger to track those hard to find mobs
3) A little halfling rogue for those otherwise impossible corpse retrievals (sneak + hide) and to sneak around investigating hard zones I might want to hunt in.
In addition it has been fun to run classes I know little about through to about 20 to learn a little "from the inside".