> If those other applications use their own local GPS clocks, what is the significance of NIST (and the 5μs inaccuracy) in their scenario?

Verification and traceability is one reason: it's all very well to claim you're with-in ±x seconds, but your logs may have to say how close you are to the 'legal reality' that is the official time of NIST.

NIST may also send out time via 'private fibre' for certain purposes:

* https://en.wikipedia.org/wiki/White_Rabbit_Project

'Fibre timing' is also important in case of GNSS signal disruption:

* https://www.gpsworld.com/china-finishing-high-precision-grou...