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."
https://wiki.openstreetmap.org/wiki/Simple_3D_Buildings#Building_outlines