Some fruit trees can only be reproduced by grafting. Grafting a fruit tree can change a tree to a new variety.
It is also a method of using a root system that is better adapted to a particular soil or climate, and create trees with particular strengths that are useful in different environmental conditions.
A fruit tree planted from seed will not become an exact replica of the original fruit tree and fruit produced.
Grafting a tree is the best way to ensure that the characteristics and identity of that tree are maintained in subsequent fruit trees, and the fruit will be exactly the same as the original fruit tree.
As an added bonus, a grafted tree will also produce fruit much faster than the trees grown from seed — often in as little as a year after grafting, but generally 2 years.