Rooms are the biggest problems we audiophiles face. Without them we'd not have great sound—just listen to an outdoor speaker—but with them we run into all sorts of problems to be solved.
Ignoring the biggest problem, bass (which we know can easily be solved with proper placement of even a single sub), the upper frequency reflections must be dealt with.
Take the POFR (point of first reflection) as an example. The POFR is to be found along the sidewall of the room. It is the first point where the sound from the speaker hits the wall closest to it. The wall distorts the audio before sending it on its merry way to your ear. Because the total distance between speaker->wall->ear is greater than the speaker->ear direct sound, that distorted sidewall reflection is also out of time, muddling the three-dimenisonal image.
An effective cure for this would be to either have no sidewalls (i.e., a big room like we had in Axpona) or a perfect absorber. Both solutions can be problematic: most people don't have/want an auditorium sized living room, and there's no such thing as a perfect absorber.
What to do?
Diffuse. Scatter the frequencies such that the ear/brain mechanism cannot accurately identify what's presented to it so that scattered sound is mostly ignored.
Having the knowledge of little tricks like this can make a huge difference in your audio system's performance.