Loopback interfaces are used to provide an interface that never goes down. Routing protocols can use loopback interfaces as support interfaces to set a router ID (in OSPF), or to advertise an IP network (in routing protocols, listing a network in the 'network w.x.y.z' statement won't advertise the network if it doesn't exist on the router. assigning the network to a loopback interface allows the IP network to exist on the router).
There are other uses too, but these are some major reasons to see them in a configuration.
