Absorption behind components and speakers rationale?

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.

Behind speakers is usually to clean bass reflections. But this means panels must be THICK (2" and up) and/or use cylindrical traps in corners

That's curious about cylindrical traps. Is there much of a difference between cylindrical and the triangle panels that fit snuggling into the corner?

The GIK Tri Traps that are triangular are somewhat effective, but nowhere near as effective as the full sized corner traps or the ASC Tube Traps (which are even more effective). With the tube traps, you are supposed to place them with a little room between the corners and the traps so that it can absorb upon the reflection of the corner, while the front half of the traps have a diffusion layer so that it doesn’t overly deaden the high frequencies. In all fairness, the Tri Traps can now be purchased with attractive diffusion panels in front, but the Tube Traps are much, much more effective. I speak from personal experience having had both and not being a dealer for either.