Why do some projects rebuild the editor, engine and everything else?

I downloaded the Chaos Destruction demo after building the 4.27 engine and compiled it with Visual Studio. IThe project rebuilt everything that was already compiled. Is this the expected way to bulid projects? I assume its because of slightly different code in the engine? Can’t I just build the necessary stuff or is all this compilation needed?

Each project is it’s own game, with everything compiled separately.