Video ‘sync’ is one or more - of a combination of field/frame sync, colour subcarrier etc- intended to provide the (much higher frequency) synchronisation of video signals required for cutting, mixing, effects...
FPS style clocks are a very coarse method of syncing. They don’t carry absolute time info, so you have to manually align the two timelines - then the relationship between the two ‘fos’ clocks is compared - which provides a means to nudge the transports back in sync.
These decks are often locked to the same crystal or house reference - so that once they are in sync, they don’t drift apart over time.
Some older style film/mag transports even have a slip clutch running off the same motor/gearvtrain!