In the story Susie is left in a void where she can not move forward nor backward. She has some things that have been left undone and one that is mentioned is her desire to have kissed. She has never had her first kiss. She was taken away by the man who killed her. In order for her to move forward and to give her that one special part of the transition from childhood into adolescence the author used the kiss as the representation of the transition.
For Susie the kiss was also a transition form the earthly things that she had to leave behind to the process of letting go which would enable her to go to her real heaven and not just the temporary place. By allowing Susie to be in Ruth's body, she could have access to the final things that kept her earth bound.