The rotary plate contacts are providing power supply for the wiper motor until
the the wiper arms are reach their "rest" position. If they would defective
the interval wiping wouldn't work, and the wiper arms probably won't able to
return to rest position.
The low and high speed supply is fed directly to the
wiper motor, and there's an independent parallel supply through the rotary contacts
towards the low speed terminal. If the low or high speed supply is interrupted
the independent supply keeps the wiper motor energized until it's reaches the initial
position (this independent supply is activated together with the ignition).
Loss of DC motor power could cause by defective winding in the wiper motor, and / or
voltage drop in the power supply path (including wires, relay contacts, stalk switch contacts).