They use it to lock you into a military-type enlistment contract for one-year stints. They do this to make sure it hurts if you quit, because they know if they don't every new manager will quit in less than six months. They can also garnish your wages at your next job, and they've been known to this in the past to recoup relocation costs.
Supposedly if you're involuntarily terminated you don't have to pay back the relocation. The easiest way to do this is to violate the attendance policy.