[fnurt]

Checking also sets you up nicely for the next street. If you get help, you can easily raise as a semibluff, and you will appear to be representing either a made hand or a strong hand like TTT or 999 that slowplayed the flop. Also, more importantly, if you raise the turn it's hard for anyone to raise you back unless they have a hand, because an obvious reason for you to take a free card on the flop is because you had a nice draw.

For example, say a J comes on the turn giving you top pair, but putting JT9 on the board. If someone bets and you raise, it's difficult for them to reraise you without a real hand, because KQ is a hand you easily could have played like this.

Note that if you check, it's important to have a plan for what happens when someone bets into you on the turn, because it's very likely to happen.