The reason for the distance and/or damping of the rear wall is to prevent cancellations. The sound reverses polarity as it bounces off the wall, and then combines with the front wave to create signal cancellations at various frequencies related to the distance from the wall and the speed of sound. Moving the speakers further away puts the cancellations lower in frequency but they will still be there. But, a lot of folk like the added reflected sound since the speakers rarely fire straight back into a wall, so some of that rear energy gets reflected around the room. Provides a "larger" soundstage though probably not true to the source and smears the image. Pros and cons...
Absorption or diffusion behind the speakers will work. Absorption is usually cheaper and works over a broader frequency range than common diffusors. I prefer diffusion but have all absorption in my room now (disclaimer: I own Magnepans, not ESLs, though the principles are the same). Damping the back wave provides a much more precise image, no (or much less) comb filtering, and allows you to place the speakers closer to the wall behind. The main con is the loss of added reflections from the room, something I personally do not care about but some (many?) may miss. Diffusion offers many of the same benefits but retains some of the reflected energy for a more spacious sound (albeit with less precise/more smeared imaging).
Anybody setting up dipoles for the first, or twentieth, time should be prepared to spend some time tweaking their positioning for the very best sound. Damping the wall behind can significantly shorten that time but it is still very important.
IME/IMO - Don
"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley