Why do people make smurfs?

The question is pretty self-explanatory. I can think of three possible reasons.

1. Main account got banned
2. They have a friend who is just starting and want to play with them without pulling the newbie up to level 30 play.
3. They want to stomp new players so they can feel good about themselves.

Am I missing any reasons? I see pro players with tons of alternate accounts but never understood.