Church membership is important to churches for a number of reasons. Some churches view their membership as the target audience for their ministry. Other churches use membership as a measure of the effectiveness of their ministry. Membership gives definition to a faith community, and for some folks, it is an important part of their personal identity.
But why is church membership important for church members? Perhaps an even more important question is: why would anyone want to be a church member? What's in it for the member?
The Book of Order of the PC(USA) has a fairly specific definition for membership (because, as always, we do things decently and in order).