One application I am familiar with is their use in ocean bottom hydrophone and geophone recording nodes for reflection seismology. Land based, transition zone, and boat tethered systems can be synced to GPS time. Battery powered ocean bottom nodes have to maintain an independent clock with a limit on the maximum amount of clock PPS drift from the moment they are deployed. The longer you want it to operate usefully on the ocean bottom, the more accurate your timekeeping clock has to be. I don't know if I would say this is a non-practical purpose, but anything I can think of to do with a highly accurate clock would have some practicality