absorption - assuming your speakers are sending acoustic energy forwards - if you have dipoles or panels and want to reduce the quarter wave cancellation (comb filtering) from reflected energy with primary energy then diffusion is also helpful.
Since all bass is omnidirectional below about 500 Hz - you always get reflections off the rear wall behind the speakers that cause quarter wave cancellation in your bass and lower midrange (peaks and nulls - looks like a comb - it makes the lower midrange sound diffuse and lacking tangibility - especially noticeable with male vocals - although some people like this effect as it makes for a bigger diffuse soundstage - the Bose 901 approach).
Since you can't really diffuse bass frequencies (wavelengths are too long) then absorption is the only option for conventional speaker set ups.
Since all bass is omnidirectional below about 500 Hz - you always get reflections off the rear wall behind the speakers that cause quarter wave cancellation in your bass and lower midrange (peaks and nulls - looks like a comb - it makes the lower midrange sound diffuse and lacking tangibility - especially noticeable with male vocals - although some people like this effect as it makes for a bigger diffuse soundstage - the Bose 901 approach).
Since you can't really diffuse bass frequencies (wavelengths are too long) then absorption is the only option for conventional speaker set ups.