Locks discourage theives, but don't actually stop them. They can break your lock; the lock only helps if it makes it more effort than it's worth, usually by making easier to go steal the one further down the rack that has no lock, or a less secure lock.

But there are two halves to the equation: the difficulty of breaking the lock, and the value of the item that's locked up. The more valuable the locked item is, the more effort is worthwhile. Thus, a rusty bike with a good lock is less likely to be stolen than a fancy new bike with a good lock