You are asking multiple questions at once.
Can it fly a sat /a rocket/a UAV?
Yes, sort of.
The Core Flight Executive is a portable, platform independent embedded system framework developed by NASA Goddard Space Flight Center. This framework is used as the basis for the flight software for satellite data systems and instruments, but can be used on other embedded systems.
(highlights by me)
There exist other packages for it, as it is a pluggable framework, that can do different tasks needed for different spacecraft. Remember, its a framework, not a completely ready-to-use package that you simply drop into your spacecraft.
What are its uses? / Where is it really used?
NASA's cFS has been used in a number of missions, starting with the Lunar Reconnaissance Orbiter which helped fund initial development (source, pdf). It has also been used in Moon Express, the prototype robotic lander Mighty Eagle, the recently launched Magnetospheric Multiscale (MMS), and will be used on the upcoming Solar Probe Plus. For more uses, see https://cfs.gsfc.nasa.gov/Applications.html and the "Featured Missions" sidebar.
What advantage does it bring for the developer community?
For the normal developer community, not much, unless you have spare money to build a spacecraft and buy a launch for it. It might be possible to study the software to learn about design techniques, but this will be of little interest for the normal software developer.