We think of circles as being closed, or overlapping, or in social terms, as small, as in a small circle of friends. However giving circles are more akin to a circle dance in some folk traditions, starting with a few individuals and widening and growing to include others as the dance continues.
Jessica Bearman, Angie Eikenberry (who are both friends) and two other authors (whom I don't know) conclude their study by observing that many giving circles form expressly to engage a "greater diversity of donors, including women and people of various ethnic and racial backgrounds and varying wealth levels, to build community and grow philanthropy" (p. 33).
They also find that giving circles "provide ample activities and learning opportunities for members beyond giving money. Many members are highly engaged with funding recipients beyond financial support, including volunteering and offering technical assistance" (p 33).
In a society in which people WANT to give but do not always know where or how, this form of collective giving can be one way to help spread the joy of giving.