Adjusting clocks that aren't atomic.

Cellular base stations used to have rubidium sources disciplined to GPS as a reference frequency for synthesis. AIUI, put the expensive, precise part in the base station, have the cheap handset lock onto that. Free accuracy. Not sure if they still do it that way. That's where all the cheap used rubidium sources on eBay come from.

They have applications in the database space, e.g. see Google's spanner: https://www.theverge.com/2012/11/26/3692392/google-spanner-a...

Low latency time source for stuff like Spanner that appreciates times that are the same across disparate data centers?