Offense can go into slumps and hot streaks, but defense is more revealing. The sport may seem to revolve around pitching and hitting, but consistent defense secures titles... and erratic D definitely loses rings.
Big league regulars need to make routine plays consistently. For a bad team trying to approach contention, shoring up the defense should be the first step -- and maybe the least expensive -- towards some semblance of reliability (especially to help questionable pitching, which can never afford to give away at bats or waste extra pitches).
For many reasons, the Sox chose to walk away from their best defensive outfield in half a century -- the one that won three straight AL Easts. They chose not to replace them with All-Star free agents, and not just Springer, but legit hitters like Brantley and Rosario. Ok... but they also passed on many low-cost Gold Glove-caliber centerfielders. Instead, the Sox opted for versatility -- guys who play a lot of positions, but few good enough to be full-timers at any one spot.