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.[1]

*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.
[1] David Sklansky (1987). The Theory of Poker. Two Plus Two Publications.