Makes it easier for your opponent to tell what your maneuver is. Look at the mark on the dial and the notch where you look at the maneuver, estimate the angle. If you know which maneuver has its back marked and you know the dial you can predict the maneuver.
It's like a clock: you can't tell what time it is on a clock with no markings. Add a marking at 12 and you suddenly can.
Edited by Lagomorphia