There are several reasons why people graft - here are few examples:
You may want to combine the beneficial characteristics of two species. For the root system portion of a plant, these characteristics may include disease resistance, wet soil tolerance, drought tolerance, and/or dwarfing tendency. For the fruit of a plant, these characteristics may include taste or flavor, sugar content, size, color, and/or yield. By uniting root system features that are beneficial to your growing area with fruiting characteristics that you desire, you may end up with a healthier, more productive, and longer-lived tree.
Some fruit varieties are not self-fertile. They require a pollinating variety in close proximity. Grafting one or more of those pollinating varieties onto the non-self-fertile tree helps increase the likelihood that your tree will yield lots of fruit.
You may want to extend the fruit-bearing season of a tree. For example, you may have a plum variety like Santa Rosa that only yields plums for 1-2 weeks in July. By grafting an early season variety and a late season variety onto this tree, you can possibly get plums from that tree in June and in August.
