What it trains
Strength through range, and control at the end of that range — the part of a movement most people are weakest in and least often train. The reformer lets load be applied where bodyweight cannot reach, particularly through the hips, adductors and deep trunk.
The adaptation is largely neuromuscular: better recruitment of the muscles that stabilise the pelvis and shoulder blade, and better tolerance of positions the body currently avoids. It also trains eccentric control, since the spring is always trying to return and you are always resisting it. Members who lift heavily tend to find it exposes a side-to-side difference they did not know they had.