Example on a cable row: 12 reps at 30 kg, drop to 22.5 for 8, drop to 17.5 for 8. One set does the work of three, in the time of one and a half.
It extends a set past the first failure point and is time efficient, which makes it a hypertrophy tool for machines, cables and dumbbells. It is not for squats or deadlifts.
Last set of an exercise only, once per muscle per session. More than that is fatigue without extra reward.