Given that backup stores are just regular folders, and that it is possible for Sender and Receiver to work on the same machine, some advanced configurations are possible.
2 (or more) friends can backup to each other. Adam backs-up his files to Bob and, likewise, Bob backs-up his files to Adam. They both need the Sender and Receiver to be installed.
By installing Sender and Receiver on one machine, this machine can backup its own files locally. This can also be considered if you want to take weekly snapshots of a Receiver; it would send its own store ‘to itself’ on a weekly basis.
Adam and Bob might both back up to a central Receiver. By installing a Sender on the central Receiver, their network administrator would be able to relay the entire backup store offsite (perhaps overnight).