There are five basic reasons to raise in hold 'em:
l. To get more money in the pot.
2. To drive players out.
3. To bluff (or semi-bluff).
4. To get a free card.
5. To gain information.
If you think you have the bettor beat, it is generally correct to raise, even if you risk driving out players behind you. In fact, in many cases, it is very important to drive out players on the flop. This is because in today's modern structure, the size of the pot is often so large in relation to the bet on the flop that it sometimes becomes correct to draw to long-shot hands if there is no raise (on the flop). Consequently, your raise in these spots often will save the whole pot for you.
By the way, if you are raising on the flop to drive people out, it usually will work only against those players who have not yet had a chance to call the original bet. It is a rare player who will fold his hand on the flop for one more bet after he already has put money into the pot. (Sometimes, however, the original bettor will help you drive out players by reraising.)
Even if you are not sure that you have the best hand, a raise is often correct. Keep in mind that if you do hold the best hand, or if your hand becomes the best hand, your raise may have stopped other opponents from drawing out on you.