Why are rootstocks used for apple trees?

Apples do not come true from seed, so when we plant apple seeds, the plants that result are not the cultivar that we obtained the seeds from. If we take six seeds from an apple and plant them, the resulting trees will produce fruit that is different from fruit on the parent tree, and the seedlings will be different from each other. To maintain a cultivar, we have to propagate the tree vegetatively. Since apple cuttings are difficult to root, we have to graft or bud a cultivar onto a rootstock.

Answer provided by Richard Marini, Penn State University.