When sound hits a rigid plane surface (like a wall) the angle of reflection of sound is equal to the angle of incidence. Using absorption material decreases the reflected sound energy but does not prevent sound to be reflect into a single direction.
A diffuser spreads the sound in a room, as shown by the next figures. These show the spatial distribution in 3D charts of a rigid plane surface and an acoustical difuser (figures takes from the RPG site).
The utility of a diffuser is to
The simplest diffuser is called a polycilindrical diffuser, which is shown below. It has great effectiveness in terms of spatial dispersion of sound but lacks temporal diffusion. It is very important to state that a diffuser works not only by spreading sound into several directions but also on reflecting sound in an asyncronous way.
a) spread the sound in the room
b) solve acoustical problems while retaining some of the acoustical charateristics of the room.
In order to provide temportal dispersion a diffuser has to have a more complex geometry. The diffuser below has different wells with different depths. Each well of the diffuser reflects sound at a specific time and therefore breaks strong isolated reflections (which we call echoes) into smaller reflections of less intensity and slightly delay from each other.
Therefore we no longer perceive sound as an acoustical flaw but actually blended together with the direct sound. Better, by redirecting sound into different directions, the diffuser helps sound to propagate in a equal manner in a room, creating a more uniform sound field when compare to the same room with rigid plane walls.
So is the diffuser above better than a polycilindrical diffuser ? It depends !
Sometimes we want just to spread the sound into a larger area but also want that the sound arrives to different listeners at the same time (for instance we may use curved diffusers in the ceiling of an auditorium for speech purposes). On other ocasions we want to divide the sound into smaller reflections in terms of temporal dispersion and we should use a diffuser with a more complex geometry.