Diffusers: under what circumstances might they help when placed behind speakers?

A diffusor placed centrally and behind the speakers is very often done in high end showrooms and private listening rooms.  Adds clarity without deadening the mid-treble range.

Thanks Erik.

I have no complaints relating to clarity, but am thinking more along the lines possibly enhancing the depth of the soundstage.

For soundstage depth improvement, try moving the speakers out into the room more. Also removing furniture and gear from between the speakers or get it as low to the ground as possible will have a huge impact.