You don't have to concentrate on proper gas/clutch control when you are already in the right gear with the clutch fully released.
That way you can focus on other things like apex, exit speed, obstacles and road surface.
The brain can only think of one thing at a time. Where there is more than one thing happening, we can give each item slices of attention. The more we have to think about any one thing, the less time we have to think about other things.
My mechanic said to do clutchless downshifts only in the winter when he needs the money--he's plenty busy in summer.
It can be learned to squeeze the brake lever and roll-on a blip of throttle at the same time for smooth downshifts while braking.