Absorption behind components and speakers rationale?

Below 200 hz or so sound waves become more modal eventually becoming omnidirectional. Speakers are usually closest to the front wall so that there is typically a lot of low frequency energy pooling in the front of the room. Thick absorption on the lower portion of the front walls and both corners floor to ceiling will help with this. Between the speakers there is always the possibility of sound waves hitting this area and causing smearing of your signal in 2 ways. One is edge diffraction where mid and high frequency sound waves can hit your front wall if the drivers are more than an inch or so from the cabinet edge especially if the cabinet isn't curved. This sound wave when it strikes the the front wall can ricochet back to your listening position and allow out of time signal to arrive slightly after the primary signal smearing the sound. You can either diffuse or absorb the area between your speakers to fix this depending upon taste. The second possibility are from sound waves reflecting off the rear wall directly from your speakers then hitting the front wall and then returning to the listening position. These will be quite weak and not terribly problematic. Also remember to measure the distance from your speaker to the front wall and determine the frequency wavelength for this distance. 1/4 wavelength of this distance will return (SBIF) and cancel that frequency. When you calculate that wavelength treat it on the front wall. Blankets etc. are not helpful as they simply turn down the tweeter as they aren't thick enough to treat midrange down. I would recommend at least 4 inches of a good absorber (not foam). True bass traps (effective below 150hz or so) need to be thicker and/or engineered specifically for the task. 1st reflection points can be diffused but you need to be 3 times the lowest frequencies wavelength and good ones are very expensive. 

@aldavis

Incredible comment and I understood what was said which is pretty remarkable for someone like me.

I think the pressure up in the front of the room and having it bounce back and degrade the signal coming from the speakers is of note. Smearing is an interesting term.

It appears as if those Bubos acoustical panels from Amazon are only 0.4 " thick.

@immatthewj

I know, so I may need to double them up.  I may need to find different ones without the full back adhesive which I found on others. 

Absorption behind speakers can reduce bass cancellation. As you move the speakers out into the room a different bass frequency is affected. The bass traps reduce this cancellation.