Your post kind of lumped betting and raising together, what sklansky says about raising should apply. He says there are 7 reasons to raise:

1. Value
2. To drive out opponents when you may have the best hand
3. To bluff/semi bluff
4. To get a free card
5. To gain information
6. To drive out worse hands when your own hand may be second best
7. To drive out better hands when a come hand bets

To shorten those Id say there are the following reasons:
1. Value
2. To drive out opponents
3. Bluff/Semi-Bluff
4. To get a free card
5. To get information