The most important reason is because this is the sure way to propagate the variety we want from tree to tree. If you were to eat a piece of fruit, a Bartlett pear perhaps, and plant the seeds from it, each seed would grow into a pear tree all right, which would bloom and produce pears, but just as ten children of the same parents can all be quite different from each other, the pears borne by each tree would all be different from the Bartlett pear you ate, and different from each other as well. The trees themselves might not even be of the same growth habit as the Bartlett tree. In order to be sure of having real Bartlett pears, we must take a piece of a Bartlett tree. This piece is called a "scion". We then graft it onto some other pear tree, where it will "do its own thing" and produce Bartlett pears.
Most people know that it is quite possible to graft scions of 5 different varieties onto 5 different branches of any existing tree of the same kind (apple on apple, pear on pear, etc.), thus making a "5-in-1" curiosity, but generally (certainly in commercial orchards) each tree is of a single variety, the scion having been grafted onto the lowest few inches of the trunk, just above the roots, of a one-year-old tree of the same kind. This baby tree of pencil diameter, which receives the scion and has its top cut off and discarded, is called a "rootstock," and it and the scion remain together all their lives. Nearly all woody plants of "named" varieties ('Elberta' peach, 'Peace' rose) are propagated in this way, not by seeds. Of course it is also possible to propagate a named variety by taking cuttings and inducing them to root, but with fruit trees this is a lengthy process, and even then the percentage of successes is not high. Moreover rootstocks can be chosen that are able to confer advantages to the final tree, which we will explain later.
The scion, as we have said, is a piece of the variety whose fruit we want. What, exactly, is a rootstock? An apple rootstock, for example, is simply another variety of apple tree, each variety usually indicated by a code like M-26 or P-22, which originally grew from a seed somewhere, was found to have something desirable as far as its roots were concerned, and is now itself propagated by a specialized nursery in one non-seed way or another ("clonal rootstock"). This tree is entirely capable of living on its own; if nothing were ever grafted onto it, it would grow into an apple tree, bloom, and bear apples, although the apples would not likely be of very good quality. But we are not interested in its more or less inferior fruit; we choose a particular variety of rootstock only for its roots, which have one or more attributes that the roots of the scion variety may lack, such as disease resistance, ability to tolerate certain adverse soil conditions, or ability to cause the resulting grafted tree to be smaller when mature ("dwarfing rootstock"). Thus we want the rootstock only for what its roots can do, and the scion only for the fruit that its branches and leaves will produce. When the two are joined, we will have a tree more desirable for our needs than either would be if growing on its own.
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