To add to bjsausts reasons to bet, I've been told by alot of good players that the when you bet you mainly want to achieve these 3 things:

1) Get better hands to fold
2) Get worse hands to call
3) Protect a vulnerable hand

Following that if you have a monster hand you want to bet for value to build a pot because your hand is soooo good that alot of worse hands are out there that stand to call you. And you are so comfortable with your hand that you are wanting to build a pot and felt with it.

When you have absolutely nothing but air you should be inclined to bet because your hand is so poor that you have no showdown value. Say you end up seeing a flop of AT6 with 24. Betting here gets alot of better hands to fold.

So I always try to run through what a bet will accomplish before I make it and if it meets some of those 3 goals then it's generally a good idea.