Use it or lose it is the easiest answer. If you took a poll then you’d find that most people do what they like or can.
This is an underrated aspect of training. There are a few people who can give 110% even if they hate the program, but most of us go all out when we like it and force it when we don’t. The difference for me is giving 110% or doing the bare minimum - like 85%. I think that’ll show over time.
Back to what you can do. Some of us have injuries and have to adjust as we age. Even if rep range A is best for this goal, rep range B might yield better results for me. If rep range A sidelines me with nagging injuries every 6-8 weeks then rep range B done without any breaks could be superior.
See how hard this is to paint into a black and white scenario? I cite this research study often - lifting to failure at 30 reps vs 8 reps resulted in the same amount of hypertrophy with 8 reps resulting in slightly more strength gains.
I would tell someone to focus on training in the 3-12 rep range. Change it up within sessions (i.e. bench 5x3, DB incline 3x8, cable fly 3x12) or in training phases.
The bottom line is that you have to use your muscle or you’ll lose it. Using it could be things outside of thy gym, too. Remaining active is using it.
Edit: I see that this is old, but it showed up on my feed and I like this discussion. This is what happens when I don’t browse the forums as much as I once did. 