Reasons to flush:
Media PPM is too high. Determined by runoff. This is an actual flush. For example, the media PPM is 2800 and you are on the verge of having a bad time. So you flush it down to 700ppm. This takes a LOT of water unless you use some type of flushing agent. About 3-5 times the normal watering volume with just pH'ed water.
'Flushing' during the last week or two of growth. Why? Because you want the plant to fade because it looks nice. You save a little money on nutes. The yield, potency and smoke quality will be the same either way, IF you timed your flush right. This is a gradual flushing in peat. The plant is still eating every time you 'flush' it until there are no nutes in the media. This takes several watering days.
What happens when you flush for two weeks, but the plant needs two more weeks to finish? Then you probably lose something compared to timing it right.