Bending the knees does some helpful things.
IMO, in areas of timing, height and balance.
Timing: Bending your knees further makes a difference when you go for 1-turn j-stick vs a 2-turn j-stick. Also gives ample time to eye up the tip if you rotate the ken slowly. Keeps the ken/tama at eye level (or near eye-level) at all times.
Height: If you want to do stuff like 3-turn lunar flips or 4/5-turn j-sticks, making the ken turn faster alone is not enough. Bending the knees helps to add more height to some tricks.
Balance: Bending the knees acts as a sort of cushion when you land balance tricks like lighthouse/bird/lunar. Some tricks have complicated, fast moving tricks and then suddenly have a bird (for example) stuck in the middle. The knees also help slow down or speed up the tempo of a trick.