Doing too much or not enough can both slow down your maximum potential rate of results.
Too much volume will take longer to recover from with little to possibly zero additional growth occurring. Taken to an extreme, you may even start working backwards if you are chronically doing more work than you can effectively recover from.
Too little volume and, well, you’re just wasting your time.
As you recover from training, eventually that window shifts.
Understanding how much volume you are currently doing allows you to adjust up or down as-needed to find that ideal window for optimal results.