You might be surprised how much "coherent" energy is radiated behind speakers. It is then reflected and out of phase with the direct sound from the speakers. Diffusors behind the speakers break up these waves and make them less coherent. You still get some radiated energy, but it's scattered, thus minimizing the "in phase out of phase" response at various frequencies at the listening position. In general the result is a cleaner sounding system because you are hearing relatively more sound of the direct component vs the radiated.
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