Plant cuttings naturally want to continue producing new stems and foliage. Successful rooting involves encouraging cuttings to stop producing stem cells and start producing root cells. The proper use of rooting hormone can speed this process up considerably.
Rooting hormones contain auxin, a growth regulator naturally produced by plants, which increases cell length. This explains why it's possible to root many plant cuttings without the use of additional hormones whose primary function is to boost auxin levels.
How Hormones Work
When rooting hormones are applied to the end of a plant cutting, its stem cells are replaced with undifferentiated cells that eventually turn into root cells. The new cells form calluses at the site of the cut, and the hormones hasten the appearance of root cells that will produce strong healthy roots.
Cell differentiation is the process that decides which plant cells will become leaves, stems or roots. Auxin reverses this process, according to the Santa Clarita Rose Society, causing all the cells to become roots. It's possible that, because plants manufacture their own auxin, the excessive use of hormones may actually slow the rooting process down or stop it altogether.