It also turns the master cylinder from a horizontal to vertical position, where they were designed to work.
I also thought it helped by moving it away from the heat of the engine.
On a T the foot pegs move the foot forwards as well as backwards, but the big thing that Griffs rearsets do is move the rear brake master cylinder onto the heel plate of the foot peg.
This moves it away from the vibration from the engine thus avoiding the cavitation in the fluid and the loss of the back brake.
I also thought it helped by moving it away from the heat of the engine.