[TrueDuality]

Another commenter mentioned that this is needed for consistently ordering events, to which I'd add:

The consistent ordering of events is important when you're working with more than one system. An un-synchronized clock can handle this fine with a single system, it only matters when you're trying to reconcile events with another system.

This is also a scale problem, when you receive one event per-second a granularity of 1 second may very well be sufficient. If you need to deterministically order 10^9 events across systems consistently you'll want better than nanosecond level precision if you're relying on timestamps for that ordering.

[cma]

Google Spanner paper has interesting stuff along these lines, heavily relied on atomic clocks