Very useful on sequencer lines, adding variation and movement. I usually control it by a LFO (have lots of LFOs, love them). If you crank up the resonance (at the full modulation point) it adds a percussion-like quality to melodic lines that comes and goes, and when used with lots of delays I think it is very nice.
Also useful on drones, where a touch of slow S&H and/or envelope control of resonance varies the sound while keeping the body of the tone.