Diffusion is the complement to absorption. While absorption removes sound energy (making a room sound deader), diffusion scatters it (making the room sound even and spacious without distinct echoes). Quadratic Residue Diffusers (QRD) use wells of varying depths to scatter sound evenly across a wide frequency range.
Professional control rooms typically use absorption at first reflection points (side walls, ceiling) for clarity, and diffusion on the rear wall for a sense of spaciousness. A completely dead room sounds unnatural; a completely live room sounds unfocused. The right balance of absorption and diffusion is the goal.