Think of it like the RPO. It is designed to keep the DE assignment sound and to not crash down on the RB. Every once in a while, the QB needs to keep it to make the DE stay put. Think Tyler Batty vs. OSU.

The Jet Sweep is designed to make the defensive secondary play assignment sound on the Jett back motion opening up room for passes and runs to other sides. Every once in a while you give it to the Jett back to make sure they stay assignment sound and you keep the openings you want in the secondary.