Coaches assign rest days because they have to. Here’s what happens during a rest day; muscles will get stronger.
It’s really very simple. An hour or two after a hard workout you’re at a minimum, in terms of fitness. Things have to recover, and it takes a while, after which you’re stronger than you were before.
Not I’m not a perfect example of always using rest time properly, especially if I feel like I’ve done a sub-standard WO and “need to get a better workout”. So once in a while (TOTALLY against my better judgement) I might add a little here or there sometime afterward. Now the REASON I had a kind of weak workout was probably because I was a little tired, had a touch of a bug, a bad day, who knows. The workout I did was actually about the best I could have done at the time, and pushing further just made it a little worse. In fact, I’ll know the next day, and maybe even the day after that, it was too much.
And this is the clincher: the workouts the day after you were supposed to rest, (and didn’t) or have an “easy workout day” will be sub-par and you won’t gain much, if anything, other than some more pain. The saying, “No pain, no gain”, doesn’t always apply.
That’s the problem with people who are driven, and triathletes are about as bad as any for not allowing a decent down time. We’re afraid we’re not getting faster if we don’t nail every workout and throw in something extra during our rest days.
Without that programmed down time you won’t get faster. You must learn to judge for yourself at times, also. If you feel you’re going to need it, TAKE IT; learn to tell the difference between just being lazy and pushing too far.