Hello,
Grafting is done for a variety of reasons:
1) Seedling rootstock is virus free - if for example you had the means to initiate a thermotherapy routine to create a virus free growing tip, you would need a virus free rootstock for grafting purposes.
2) Propagation of a particular variety although rooting cuttings is very efficient for our purposes. This is actually more important for propagating varieties of other types of fruit trees such as plum, pear, apple, etc.
3) To take advantage rootstock's desirable characteristics - a) nematode resistance or immunity b) vigor control or dwarfing characteristics
4) For fun - one might to create a tree with multiple varieties on one trunk.
5) Propagating a "rare" variety. If you are comfortable with grafting and were, for example, having issues rooting a variety, this might guarantee that a variety is propagated.
There might be other reasons but those are ones that I can think of.
Ingevald