Composition doesn't happen only while you're shooting. Using the crop tool in your photo editor, you can reframe your shot later, effectively recomposing it. Through cropping, you can eliminate clutter or rebalance your image. Sometimes you'll shoot with the idea of cropping later, because you simply can't get the shape you want in-frame (for example, if you want to shoot a really wide landscape).
Whenever possible, though, you should always try to compose correctly in-camera--if you have to crop later, you'll be throwing away pixels, and therefore reducing your maximum print size.
If you're cropping a photo in an image editor, follow a few basic cropping rules and tips.