It can take a long time to turn some capacity on or off, and not all generators throttle cleanly to zero. Hydro plants can have rough running ranges that cause

It can take a long time to turn some capacity on or off, and not all generators throttle cleanly to zero. Hydro plants can have rough running ranges that cause excessive wear. or might have run-of river flow requirements. Coal plants have huge parasitic loads in the crushers and pulverisers and blowers, and/or poor ability to throttle below a minimum limit. For any of those reasons, it may make sense for a generator to pay someone else to temporarily take their excess capacity, rather than shut the generator down.