I've wondered whether you could spend the rest time for one muscle group by doing an exercise for a different muscle group. Is that a no-no, or is it not done for practical reasons (the logistics of jumping back and forth between machines)?
Your brain needs recovery time.
To clarify, two things.
The brain does not directly drive the muscles. It does not have copper running to them, providing a charge. Instead, cell after cell transmits the signal, from brain to muscle, each cell providing its own charge. The brain expends more energy thinking about abstract ideas, than telling a muscle to move.
Second thing, healthy, fit people run for hours, bike for hours, without issue. As a teenager, I'd work on a farm for 10 hours, picking fruit, throwing bales of hay, digging, on and on without tire.
Yet just continually walking, uses more "brain telling muscles what to do", than any single heavy lift plus even perhaps 10 seconds of break.
Realistically, writing cursive with a pen, typing fast, should use as much brain energy, for fine, detailed control requires many different muscles all being used at once, hence more signaling.
Your nerves don't care how heavy something is. Fine muscle control is just as intensive as anything to the nerves. Talking uses waaaay more nerve and muscle control than lifting heavy weight.
Breaks are required so that the blood can carry away waste, and replenish glucose and oxygen. That's all.
The ability to focus without fail is not unlimited. When you're holding more than your bodyweight on your shoulders, the demands to simply maintain balance are significant. This means a lot more than just muscles holding a static position. If you are not very focused on the sensations which indicate a change in balance somewhere, you will lose form and possibly hurt yourself.
Whatever the mechanism within the brain, the breaks are also necessary there. Of course the muscles need recovery time, but the brain does also.
What I have anecdotally observed is that regularly lifting closely to your maximum (particularly at higher levels) will inevitably lead to accumulated fatigue. Whatever the root cause of that fatigue is (whether muscoskeletal, endocrine or nervous) is a factor that does lead to degraded performance over time.
The literature is lacking in longer-term studies, so I'm curious to see whether my anecdotal experience is pure confirmation bias or there is a mechanism behind it.