Sculpture is tricky in that it need not serve any specific task other than to scratch some elusive itch. Good sculpture works from all angles, and is built to weather well. The more a piece is tempting to be touched, the higher the stakes and bigger the reward. And in a pinch, some sculptures can be called paperweights, giving them brief functional credo.
Lighting is a subset of sculpture but has its own rules which I haven't read yet.