A little bend in the structure shows up everywhere
Modern cars are built around a unibody, one welded shell that holds the suspension, the doors and the crash structure in exact positions. Take a hard hit and that shell can shift even when the outside panels look nearly fine. You find out later: a steering wheel sitting crooked, tires wearing on the inside edge, a hood gap wider on one side.
Worse, a shell that has moved will not protect you the way it was engineered to in the next collision. Crush zones are designed to fold in a set pattern, and a car with a bent structure can fold somewhere else. This is why structure gets checked on any serious hit, before a single new panel is hung.
