You do a set to provide stimulus to the target muscles. When the muscles are tired, they can't lift as much and each set provides less stimulus but still causes the same amount of fatigue. Eventually you're too tired to stimulate growth, but you can still wear yourself out.
> doing 10 exercises a session is a bad idea
Whats the reason for this? Time wise it seems annoying, but I vaguely recall other reasons?
You do a set to provide stimulus to the target muscles. When the muscles are tired, they can't lift as much and each set provides less stimulus but still causes the same amount of fatigue. Eventually you're too tired to stimulate growth, but you can still wear yourself out.