Do we need it? No! Do we want it? Yes!
Mainline Linux can do things really fast most of the time, but every so often, you get that outlier that could cause a large delay. Ever see your mouse jitter? With a properly setup PREEMPT_RT kernel, your desktop experience can be made much smoother.
Presumably it could provide a better VR experience without interrupts in latency.
Google is seriously developing a real time OS, maybe they're on to something
It would probably make sense to have this as an optional kernel to begin with.