Schwarber is only 24, but the former catcher turned outfielder may very well be on the express train to DH-ville unless he shows the Cubs that he can be a regular defender. And the Cubs — who may very well look to trade Schwarber in 2018 — will likely want to show potential trade partners that he can play defense too, so as not to limit his market.
Above all else Schwarber needs to show everyone that he can be a more consistent hitter. His terribly slow start in 2017 caused him to be demoted to the minors for a time, though he hit much better once he was recalled. If conditioning helps that, all the better.