Reasons to raise
Unlike calling, raising has an extra way to win: opponents may fold. An opening bet may be considered a raise from a strategy perspective. David Sklansky gives seven reasons for raising, summarized below
To get more money in the pot when a player has the best hand: If a player has the best hand, raising for value enables him to win a bigger pot.
To drive out opponents when a player has the best hand: If a player has a made hand, raising may protect his hand by driving out opponents with drawing hands who may otherwise improve to a better hand.
To bluff or semi-bluff: If a player raises with an inferior or drawing hand, the player may induce a better hand to fold. In the case of semi-bluff, if the player is called, he still has a chance to improve to a better hand (and also win a larger pot).
To get a free card: If a player raises with a drawing hand, his opponent may check to him on the next betting round, giving him a chance to get a free card to improve his hand.
To gain information: If a player raises with an uncertain hand, he gains information about the strength of his opponent's hand if he is called. Players may use an opening bet on a later betting round (probe or continuation bets) to gain information by being called or raised (or may win the pot immediately).
To drive out worse hands when a player's own hand may be second best: Sometimes, if a player raises with the second best hand with cards to come, raising to drive out opponents with worse hands (but who might improve) may increase the expected value of his hand by giving him a higher probability of winning in the event his hand improves.
To drive out better hands when a come hand bets: If an opponent with an apparent come hand (drawing hand) bets before a player, if the player raises, opponents behind him who may have a better hand may fold rather than call a bet and raise. This is a form of isolation play.
Reasons to call
There are several reasons for calling a bet or raise, summarized below.
To see more cards: With a drawing hand, a player may be receiving the correct pot odds with the call to see more cards.
To limit loss in equity: Calling may be appropriate when a player has adequate pot odds to call but will lose equity on money contributed to the pot.
To avoid a re-raise: Only calling (and not raising) denies the original bettor the option of re-raising.
To conceal the strength of a player's hand: If a player has a very strong hand, he might smooth call on an early betting round to avoid giving away the strength of his hand on the hope of getting more money into the pot in later betting rounds.
To manipulate pot odds: By calling (not raising), a player offers any opponents yet to act behind him more favorable pot odds to also call. For example, if a player has a very strong hand, a smooth call may encourage opponents behind him to overcall, building the pot. Particularly in limit games, building the pot in an earlier betting round may induce opponents to call future bets in later betting rounds because of the pot odds they will be receiving.
To set up a bluff on a later betting round: Sometimes referred to as a long-ball bluff, calling on an earlier betting round can set up a bluff (or semi-bluff) on a later betting round.