

Raising Gives You Two Ways to Win
Aggressively betting and raising is usually superior to passively checking and calling. One reason for this is that raising gives you two ways to win, while passive play supports only one.
Suppose you have 8-8 and raise before the flop. One player calls your raise and both blinds fold, but the flop of J-7-5 misses you completely. If your opponent checks, you should bet. Here's why. The flop figures to miss him too, and if he entered the pot with two big cards, the odds are against his hand containing a jack. Your pair of eights is probably in the lead at this point.
If he does have a jack, you are a big underdog. But if he doesn't have a jack, and doesn't have a pocket pair of nines or larger, you are in the lead and your bet is an offer to your opponent to fold his hand.
If your opponent came out betting, you still might want to raise. In a heads-up encounter, the flop figures to miss both of you. Your opponent might bet to drive you off the pot. But if you raise, you've put the ball back in his court and now he has to decide what to do.
If he checks the turn you'll stand a good chance of taking the pot with a bet. You might have the best hand; you might not. It doesn't matter. The trick is in the betting. You make it difficult and expensive for him to keep calling with what he suspects is now the worst hand, and by doing so you give yourself two ways to win. You can win by convincing your to fold, or you can win by having the best hand - which is the only way to win if you tend to play passively.
By betting and raising you charge your opponent a price to look at another card or give him an opportunity to fold. And that's just what you want to do. Your hand is not strong enough to lure an opponent into calling. It's the kind of hand that should be happy to win the pot right now, and the way to do that is to get your adversaries to fold. But they won't fold if you play passively. You have to be aggressive to convince them that folding is their best or wisest choice.
Convincing them that you have the strongest hand requires your willingness to commit money to it. With a middling hand, the best way to do that is through aggressive play.
For the cost of a single bet or raise you have a unique opportunity that you can never have by checking and calling. You can win in either of two ways, and if a good chance of winning only costs one incremental betting unit, the price is usually a bargain.