Let's take a look at the main reasons you have to call:
You want to see more cards – your pot odds might raise by calling
You want to limit loss in equity – you can call if you have good pot odds to call however you will lose equity on the pot
You want to avoid a re-raise – calling will prevent the player who has raised to raise again
You want to conceal the strength of your hand – if you have a really good hand you might want to call to deceive the other players into thinking they have better hands and putting more money into the pot.
You want to manipulate pot odds – when you call you give the payers that are behind you more favorable odds to call. In limit games having a big pot in early betting rounds will induce the other players to call bets in future betting rounds.
You want to set up a bluff on a later betting round - if you call an an early betting round you can set up a bluff on later betting rounds
Now let's take a look at the reasons you would have for raising:
You have a great hand and you want to bring more money in the pot
You have the best hand in the game and you want to drive out other players – if you have a made hand you can protect your hand by driving out opponents with drawing hands
You want to bluff – if you know you have a weak hand you can cause a player with a better hand to fold
You want to get a free card – if you raise with a drawing hand the player you're playing against may check on you on the second betting round and this will give you the chance to get a free card so that you can improve your hand
You want information – If you're not sure of the strength of your hand you can gain information about the strength of the other player's hand. Pay attention if you are called or raised.
You want to drive out worse hands than yours when your hand is not exactly great – the worse hands might improve and overpower yours just as if you drive more people out you have more chances to win
You want to drive out better hands when a draw hand bets: If a player with an apparent drawing hand bets before you and then you raise the players behind you who just might have a better hand than you do can decide to fold rather than call and raise.
