Grafting is a practice that is popular among commercial farmers both from a profit standpoint and from a convenience standpoint. Grafting can also be used by gardeners to create unique plants.
Some trees end up developing more resistance to diseases and adverse conditions than other trees. This disease resistance and hardiness is transferred from the rootstock (the plant being grafted onto) to the scion (the plant being propagated).
Trees that have been grafted reach maturity sooner than trees that weren't grafted, leading to blooming flowers and fruit much earlier.
Grafting is more labor intensive, increasing labor costs. But grafting is cheaper overall because the cost of equipment is lower than with growing a plant from a seed.
Much of the quality found in the fruit, leaves and flower are retained, while these qualities are sometimes lost in plants that are grown from a seed.
Grafting can be used to make plants larger or smaller than what they were meant to be genetically.