Crown molding, like a crown, rests atop something, in this case a room where the walls meet the ceiling. Painted a contrasting color to the ceiling and walls, it creates a neat and tidy frame for the entire room.
There was a time when interior residential design presented every room enclosed in its own walls with a door. Heating would have been one consideration, with fireplaces or heat sources in each room before central heating could reach all the spaces.
