- extend or improve coverage of an existing Wifi network
- with or without wifi enabled, it can provide additional NAS storage for shared files, archive data, media files or network backups
- upgrade the HD to 3TB drive for much more storage than originally provided
- provide Ethernet ports in a location of the house too far to be serviced by an existing router
If you are running mavericks, you could set the iMac TimeMachines up to alternate between the new TC and old TC. That provides you a TimeMachine backup.
Otherwise its just a NAS, ethernet switch, or another wireless access point, perhaps a guest network.