The Body, along with the Heart and Soul, is a part vital to the complete existence of any living thing. The body gives the heart and soul a vessel, granting a person a physical appearance and a form. The body is a medium for a person's heart and feelings to be expressed in the physical world.
The Heart controls the body, much like the brain in the real world, so when someone becomes a Heartless, and their heart is released, they leave behind their body and soul, which is an empty shell and usually withers away. However, if that person's heart was strong enough, the soul will take control of the body and the shell will start to act on it's own. A body being controlled by a soul with no heart is a being called a Nobody.
While the Heart is the deciding factor as to if a person can wield the Keyblade, the body can be used in a loophole for acquiring the Keyblade, as seen with Ansem, Seeker of Darkness and Riku in Kingdom Hearts. Xehanort's Heartless possessed Riku's body, therefore by controlling the heart and giving Xehanort's Heartless access to Riku's Keyblade-wielding powers.
