According to tradition, by spinning a prayer wheel even once . . . one is connecting oneself with the vast vows of the buddhas — touching or entering a river that flows to the ocean of enlightenment.
Most prayer wheels are filled with many copies of the mantra of Avalokiteshvara, the Buddha of Compassion: Om mani padme hum. So, much of the spiritual power of the prayer wheel derives from the power of his mantra, his compassionate realizations, and his past vows.
— Lorne Ladner, The Wheel of Great Compassion by Lorne Ladner, editor