Why do some people play up in USCF tournaments?

1. To face stronger opponent and gain experience?
2. To gain rating?
3. For the fair of loosing rating in own group?
4. The feel they are stronger than their current rating?

Probably all those things.

They shouldn't be able to gain rating points.  

People I know who play up want to play better opponents.  My opinion is that you are better off learning how to win against opponents in your own class.

Also, when you play in your own class you have a chance to finish "in the money". You may not need the money, but having the experience of playing a high stakes game is very valuable.  You are unlikely to get that experience when you play up.

I play up because the quality of chess is greater. The stronger my opponent the better the post-mortem. Two weak players making equally weak moves aren't worth studying. What's to learn? However when you play up its like taking lessons for cheap. And of course I deserve Elo points for my victories. If the player is better than me then they get an easy? win and closer to prize money. They also get rating points so I should too.

I came to know a fellow player that the main reason he plays up is not he wants a strong opponet but he wants to avoid facing weak opponent. These 2 may essentially sound same but there is a difference. If you are looking for stronger opponents that means you are not even okay with opponenents of equal strength. While if you are avoiding weaker opponents that means you are okay with either equal strength opponents or strongers ones.