Sync²
The standard application is to sync a software package to a drum machine or "groovebox" using sync². This way the drum machine can keep the beats flowing while you load up the next track and then you are able to start the new track on the computer without the need to stop the clock source but everything remains in perfect sync.
There are many other applications where a device needs to stop its sequencer (loosing sync) to load new data/tracks, MPC's and other sampling workstaions for instance.