Most people are very distracted in not only their daily lives, but also their training sessions. We try to convince ourselves that we're really good at multi-tasking and that the texts, tweets, and emails we send between sets doesn't impact our performance. Unfortunately, even if the quality of work is high, a lot of people fail to realize that the 60 seconds between sets has actually been six minutes.
There are some remarkable benefits to simply increasing the amount of volume you can do... and actually doing it. But the only way to make this happen without spending all day at the gym is to make your workouts more dense: do more work in the same amount of time.
A stopwatch keeps you honest. It's like a training partner that tells you to quit dragging your heels. Use it to time your rest periods. Or time your whole workout. Did it take you 60 minutes? The next time you do that workout, see if you can do it in 55 minutes. (You probably can.)
Any time my training slips, it's because I find myself doing "other stuff" between sets when I should be getting in some filler mobility work, doing an antagonist pairing, or getting my mind right for the next set. A stopwatch prevents you from drifting into "screwing around" mode, and that's why it's a five dollar game changer for your training.