As far as I can understand, this is for Linux becoming an option when you need an RTOS, so for critical things like aviation, medical devices, and other such systems. It doesn't do anything for the common user.
The sort of thing it could help with is servicing hardware that does run hard realtime. For example, you have an RTOS doing direct control of a robot or medical device or whatever, and you have a UI pendant or the like that a user is interacting with. If linux on that pendant can make some realtime latency guarantees, you may be able to simplify communication between the two without risking dropping bits on the floor.
Conversely, for the common user it could improve things like audio/video streaming, in theory but I haven't looked into details or how much trouble there is currently.
RT Linux might not pass on some certifications, but there’s likely many systems where it would be sufficient.