When you play poker, be it online poker or a poker game in your garage there are generally five reasons why you want to raise the pot:
1. To get more money in the pot
2. To drive players out
3. To bluff
4. To gain a free card
5. To acquire information
Let's look at these in more detail:
1. You want to get more money in the pot when you are in a position of strength. If you are holding the "nuts" and cannot be beaten regardless of future cards it is obvious that you want to increase the pot size as much as possible. Online poker rakes can be much higher than card room rakes, so you must try to maximize wins.
2. You want to drive players out when you hold a strong hand at that moment, but you know that future cards could still see you lose the hand. For example: you flop top pair but there is a flush draw and a straight draw on the board. You have to try and chase those drawing hands out and win the pot while you still hold the best cards.
3. Sometimes you want to raise from a position of weakness, especially if you have good position to steal the pot. This is known as a position bet, but in online poker where players tend to be weak and loose this bet will frequently not work. You cannot bluff bad players. That's a key rule in all forms of poker.
4. If you have a drawing hand and want to buy a free card it is good strategy to raise the pot. This is especially true in limit hold em after the flop. On fourth street (or the turn) the betting limits will double, but if you raise on the flop it is likely that everyone will check to you and you can get away with a free card, again having good position (last to act) helps a lot here.
5. A raise is the best way to find out what other people are holding. If players simply call it is likely that they are trying to improve their hand and are still behind you. If someone re-raises and comes over the top you should realise that his hand is strong, and possibly stronger than yours (at least he thinks it is). Again, it is a useful tactic right after the flop to see what you're dealing with before the betting limits get too hairy.
