To answer you need to understand a little bit about how teeth move… Basically the teeth are surrounded by a bunch of rubber bands (ligaments.) When you move a tooth using braces you put pressure on a tooth and pull/push it in the direction you want it to go, stretching out the rubber band on one side and collapsing the opposite side.
This pulling causes the bone to dissolve on one side and form on the other. These rubber bands have a great memory and the bone process is quite slow! If you don’t wear your retainers after completion of orthodontics you are certain to get relapse. It takes 6 months for the bone to remodel, even after that those stubborn rubber bands have memory of their previous position. If you want to be absolutely certain that the teeth don’t migrate, you have to have a retainer that you can use periodically to hold them in check!