It may have to do with recent research along the lines of, "when you really focus on the muscle group you are working, rather than tuning out or going through the motions, you can record bigger improvements in the strength gained in that muscle. The mental focus contributes to strength gains."
It may also, secondarily, help to develop muscle memory when you come back to these exercises after a break.
It makes sense: you do 100 reps of just one exercise, in very focussed fashion, you are solidifying connections between the brain and the muscle way more than can be achieved with "10 reps-throw my weights down-and off I skip to the shower"!
THese are my humble offerings on the subject.
Clare