Are you asking why anyone needs an RTOS? It provides a very robust solution that sits somewhere between running a plain old OS like Linux and running baremetal C on a microprocesser. You get the speed benefit of baremetal but also get a scheduler and a whole bunch of multiprocessing structure to run lots of applications at once without worrying about manually switching or trying to create a massive main.c to do everything. VxWorks specifically runs on lots of industrial computers such that tasks can get done in a timely manner with little overhead.
VxWorks powers the Mars Rovers Sojourner, Spirit, Opportunity, Curiosity, Mars Reconnaissance Orbiter, and SpaceX Dragon. VxWorks also powers BMW iDrive, the Apache Longbow helicopter, and the Apple Airport Extreme and Linksys WRT54G routers. It's in a lot of things and what it's not in, FreeRTOS is probably the RTOS of choice.
They won't be using "off the shelf" Ubuntu or Fedora... this'll be the linux kernel with RTOS patches (either done on their own or using something like "RTLinux/Wind River Linux").
VxWorks powers the Mars Rovers Sojourner, Spirit, Opportunity, Curiosity, Mars Reconnaissance Orbiter, and SpaceX Dragon. VxWorks also powers BMW iDrive, the Apache Longbow helicopter, and the Apple Airport Extreme and Linksys WRT54G routers. It's in a lot of things and what it's not in, FreeRTOS is probably the RTOS of choice.