The workers can't do things like go to school or do anything productive, so they're stuck at that job for a much longer time than if they could better their circumstance.
It also gives the employers the ability to point towards any deviations from being available on short notice as a reason to legally fire people. You either give up your life to your low level job or you provide ammo for them to remove all of your legal protections because you aren't being fired for a legally protected reason, but because you violated policy or weren't flexible enough