The intended use case is people trying to write realtime systems - that is, systems with a latency guarantee, like antilock brakes or flight control systems - in UNIX.

However, no sensible person would write such a thing in a general-purpose multi-user operating system. Realtime is hard, see the Toyota mess: http://www.viva64.com/en/a/0083/

"Move fast and break things" is not a great motto for automotive control software.