Wet and dry mix would be if you were patching an effect directly into a channel, and EVERYTHING was going through this, like on a guitar stomp box. You just want a little reverb.
For Bus effects, set it to all wet, so that when you put your source channel AUX slider up, and the sound comes back in on the Return track, all you're adding is pure reverb.
The fader on the return track just makes it easy to use one reverb for a bunch of tracks, in varying amounts for each. And then tweak the overall wetness.
Somehow I think you know all this. So maybe I'm not understanding the question.