Others have nailed it - it's all about the chainline. Germany_chris posted a link to the Sheldon Brown article. You need to know what the chainline should be for your bike. It's potentially one of the most important things to get right on a bicycle. If the chainline is wrong, you're setting yourself up for shifting problems, noisy drivetrain, compatibility problems, and other related issues. Unfortunately it's not just the BB that determines that - it's the combination of BB and crankset. Change the crank and you often need to change the spindle length to get your correct chainline back.