Sound Absorption Behind and Between the Speakers?

@hilde45, the reason for treatment on the front wall is to control first reflections and then help on the axial direction.
Absorbtion and Diffraction (either / both) works above 500 Hz.
Bass Trap Placement is at corners and at ceiling / floor / wall intersections.
A lot is happening in the immediate space around a loudspeaker beginning with radiation pattern / edge diffraction / port energy / cabinet /
stand resonance.
Drywall has a relatively high absorption coefficient, foundation walls are well damped but highly reflective. 
Reflections aggregate / accumulate and decay in time domain (waterfall plot).
Look for a balance ...

I once had a fireplace in the middle behind my speakers with glass doors on the enclosure - - it did great damage to the soundstage due to first reflection point issues.  Absorption worked for me then for dedicated listening sessions (but really did not work for my SO!).  Now in a dedicated room I have a wall with GIK panels behind the speakers and I enjoy a deep and expansive sound space.  Maybe deflection would work even better, but I love my art panels - - much more interesting to look at for me!  I also employ absorption at the side wall, ceiling, and floor first reflection points.

@rego and others,

Rego pointed out that:

the reason for treatment on the front wall is to control first reflections and then help on the axial direction.

If the time it takes for the first reflection behind the speakers is longer than the time it takes the sound from front of the speakers to my ears, why would I need to treat them?

Smearing is the term of reference that I see used ... the reflections still have energy ... in a room that is very 'bright' this energy is at higher levels longer.
This is about reinforcement and interference interactions.
The reflections do not stop at the first reflection but go on to two, three and lot more.
These are all ripples at many frequencies in the pond that is the listening space ...

Though the drivers appear as though only front firing, the sound produced is somewhat omnidirectional, especially as frequencies decrease. Typically  speakers are placed so that the wall behind them is closest. The ear/brain mechanism can discriminate between direct and reflected sounds so long as there is about 5 ms between them. If the sound bouncing off the wall behind the speakers (typically the closest wall) is too close, then there isn’t enough time between the direct vs. reflected sound and the sound converges to smear the perceived direct sound. Hence, sound treatment behind and between the speakers.