A controlled eccentric (lowering) phase of 3-4 seconds forces your muscles to work harder and eliminates momentum. When you slow down the rep, you instantly feel which muscle is bearing the load.
If the weight drops faster than 3 seconds on the way down, your muscles are not controlling it. Gravity is. That means less time under tension, less mechanical stress on the target muscle, and less growth stimulus per rep.
Slowing down also improves proprioception. You learn where the tension shifts during different parts of the range of motion. This awareness carries over to heavier sets where conscious control matters less.
The rule: if you cannot control the weight for 3 seconds on the way down, it is too heavy for that exercise.