[W-_-D]

An interesting use case I saw was on a complex VoIP system with lots of internal message passing, following an actor model. There were tens of thousands of unit tests to assert all kinds of crazy interop scenarios, and the system clock was mocked out globally.

So you could set up multiple incoming streams, advance the clock to a very high level of precision and assert e.g packet A triggered packet B after 20ms, or timer X fired after 1s. This made the tests extremely reliable and fast to run, since you were pretty much CPU bound - something that I don't think would have been otherwise possible.