If the Jets were sold one way or the other with Darnold, I think he would be sitting out. If they believed in him as the QB of the future, he would be on IR, rest your shoulder, see you next year. If they were done with Darnold and knew they were moving on, they would sit him so he doesn't reinjure himself and hurt his trade value.
Since they're on the fence, they want to see how he looks down the stretch. If he's impressive enough, he becomes a strong option to debate against Fields and Wilson.