I think its basically a transparent screen so it could add a photo style layer over reality that could act as a normal monitor or "edit" the look of the real world or add a monster into that shadow or that doorway etc
This article does a bad job explaining what these are to people who have not heard of castAR yet. Basically they only work when looking at a special material, which acts similar to the reflectors you see in traffic signs and on road worker's uniforms where it is brightest looking if your point of view is close to the same angle as the source of light. The projectors send an image to the material and the material reflects the image back to the eye under that projector, filtered by polarized lenses like most 3d. So it shows you a hologram, but only in the given area, but you could just cover a room with the material if you wanted a holodeck type deal. The most obvious implementation IMO would be hologram tabletop games similar to dejarik, the game R2 and Chewie play in a New Hope.