You can apply positioning to audio busses and to auxiliary busses (but not master busses). To do so, first open the Property Editor for the bus. Then, in the Positioning tab, enable Listener Relative Routing, and set Speaker Panning or 3D Spatialization. For details on those parameters, refer to Positioning Tab: Audio and Auxiliary Busses.
Positioning gives audio designers more control over sound propagation. This could be used to add realism as in the following example scenarios.
Virtual Surveillance or Loud-Speaker System: A system in your game allows players to snoop on other players with a hidden microphone. The audio is captured and retransmitted on the other end of the map from a virtual loudspeaker. After applying a lo-fi effect, the submix is spatialized to sound as if it were emitting from the loudspeaker.
Acoustic Portals: You are making a virtual reality game with a heavy emphasis on spatial audio. It is of utmost importance to simulate acoustic phenomena realistically and convincingly to maintain the illusion. You want the listener to hear the reverberated sound coming from an adjacent room. All sounds in the room are mixed together, fed into a reverb unit, and then the output of the effect is spatialized and positioned as if it were coming through a doorway.
Early Reflections and Other Effects: Sometimes applying an effect per voice is too cumbersome, but applying it as a send effect on a shared bus is too broad to be useful. Really, you want to create an instance of a bus for each sound emitter on which you can apply your effect. Spatialization can take place downstream of the submix, or perhaps your plug-in takes care of it. This is the strategy that Reflect uses to generate a unique set of early reflections for each sound emitter, relative to the position of the listener.
Clustering Sounds to Reduce Overhead: You have a vehicle sound that is made up of a significant number of component sounds driven by a complex RTPC system. The components originate from different physical locations around the vehicle, so you use individual game objects for sound emitters. However, it is costly to apply effects and spatialization per individual emitter, so you want to do this only when the listener is suitably close by. If the listener is far enough away that the subtle changes in angle are not noticeable, you can mix all the sounds together before applying processing and spatialization to the entire group of sounds.