Why
You can move the membership of one small group to another small group. For example, suppose you want to promote children in a Sunday School class that you're tracking in Membership Plus. You would start by moving the highest class level (12th grade) to the Young Adult Sunday school class. After you moved the former 12th graders, the 12th grade class would be empty, and you could move the students from the 11th grade Sunday school class to the 12th grade class, and so forth.
You can also copy the membership of one small group to another small group. Unlike moving the membership of a small group, copying the membership of a small group means that all of the individuals are listed as members of both small groups.