Supposedly its the "correct" way to tag a building outline as unused for 3d rendering. Otherwise, the building parts are consumed by the extruded outline, negating the purpose of having building parts in the first place. A 3d renderer has no idea which building parts belong to unless there's a relation. 95% of NYC's buildings using parts don't do this though, so 3d rendering is a real broken mess.

"When a building has any building:part= areas, the building outline is not considered for 3D rendering."
osm.wiki/Simple_3D_Buildings#Building_outlines

Fair enough. This seems to be how the majority of NYC downtown works with building parts though

Just adding min heights such that parts don't vertically overlap (based on the existing heights). Where one building part ends, another begins. Many buildings do parts this way.