According to the lineage texts on prayer wheels, prayer wheels are used to accumulate wisdom and merit (good karma) and to purify negativities (bad karma). In Buddhism, Buddhas and Bodhisattvas have created a variety of skillful means (upaya) to help bring practitioners ever closer to realizing enlightenment.
Prayer wheels are used every day, sometimes for hours on end. Worshippers turn prayer wheels to accumulate merit, help all beings in the world, and purify their karma (intentional actions). They are part of a meditation practice.