OK, I have just a started using Reaper, after using a stand-alone Boss recorder to do my mixing.
I tried setting up a Reverb Bus track but the output sounded terrible - all the instrument sounds jumbled like you were listening to a band in the back of large hall with no acoustic treatment. Obviously I can change my 'room' setting, but this is not the effect I wanted.
I deleted the bus track and set individual reverb on the tracks and like the effect much better.
So, why use a Reverb Bus?