Every interpersonal e-mail message creates, or sustains, or alters the membership of a group. It happens so naturally that we don't even think about it. When you're writing a message to Sally, you cc: Joe and Beth. Joe adds Mark to the cc: list on his reply. You and Sally work for one department of your company, Joe for another, Beth is a customer, and Mark is an outside contractor. These subtle and spontaneous acts of group formation and adjustments of group membership are the source of e-mail's special power. Without any help from an administrator, we transcend the boundaries not only of time and space but also of organizational trust.
An ad-hoc group convened by e-mail dissolves unless membership is reaffirmed by each message. This is a feature, not a bug. Many of the groups that perform work in a modern organization are transient. A hallway conversation is over in minutes; a spontaneous collaboration can last a day; a project may take a week. Software that requires people to explicitly declare the formation of these groups, and to acknowledge their dissolution, is too blunt an instrument for such ephemeral social interaction. Like an operating-system thread, an e-mail thread is a lightweight construct, cheap to set up and tear down.