What warm-up sets are for
Warm-up (or ramp-up) sets are lighter sets you do before a working set. They prime the movement, groove your technique, and get blood into the muscles — without adding fatigue that would cost you reps on the sets that actually count.
The idea is simple: start light and build up. As the weight rises toward your working load, the reps come down, so you arrive at the first hard set prepared rather than tired.