I'll give you an example. I'm on lead on Princely Ambitions, a 5.9 at Index. I am 10 feet above my last piece and I slot a nut. Now I traverse up and to the left. I didn't set the nut hard enough and the rope drag pulls the nut out. Now I'm 15 feet above my last piece, a yellow TCU behind a flake that might hold or might not hold if I fall. What to do? Sack up and pull the next move? I ended up reversing a couple moves and putting in another nut, setting it tightly this time.
All these machinations use up physical strength and mental energy. Anyway, I pulled the mantle without falling and the nut stayed in place.
If this climb were bolted, it would be trivial.