Prayer Wheel Consecration ceremony at Land of Medicine Buddha.
Lama Zopa explains the benefits of prayer wheels.

Excerpt From 1994 Prayer Wheel Consecration at Land Of Medicine Buddha

"One of the Benefits of the prayer wheel is that it embodies all the actions of the Buddhas and bodhisattvas of the ten directions. To benefit sentient beings, the Buddhas and bodhisattvas manifest in the prayer wheel to purify our negative karmas and obscurations, and to cause us to actualize the realizations of the path to enlightenment.
All beings (not only people but also insects) in the area where a prayer wheel is built are saved from rebirth in the lower realms: they receive a deva or human body, or are born in a pure land of Buddha.
If you have a mani prayer wheel in your house, your house is the same as the Potala, the pure land of the Compassion Buddha. If you have a prayer wheel next to you when you die, you don't need phowa. Having the prayer wheel itself becomes a method to transfer your consciousness to a pure land. simply thinking of a prayer wheel helps a dying person to shoot the consciousness up the central channel and through the crown to reincarnate in the pure land of Amitabha or the Compassion Buddha.
Simply touching a prayer wheel containing 100 million Om Mani Padme Hung mantras accumulates the same merit as having recited 100 million Om Mani Padme Hungs. Turning the prayer wheel once is the same as having done many years of retreat.
There are earth, water, fire and wind prayer wheels. With the water prayer wheel, the water that touches the wheel becomes blessed. When the water goes into an ocean or lake, it carries the power to purify all the billions of animals and insects there. A fire prayer wheel is turned by the heat if either a candle or electric light. The light that comes from a prayer wheel then purifies the negative karmas of the living beings it touches. It is similar with a prayer wheel turned by wind. The wind that touches that prayer wheel is blessed by the power of the prayer wheel and then has the power to purify the negative karmas and obscurations of any being it touches."

Lama Zopa Rinpoche