Some guns need the bolt rebound for proper cycling.
The M14/M1A is a good example of that. They can't use a bolt recoil buffer, so the ones you see for sale are simply oprod buffers.
Some guns should have had buffers in them from the day they were made. The Ruger 10/22's bolt stop pin is steel, sitting in an aluminum receiver. Repeated hammering of the bolt stop pin by the reciprocating bolt will wallow out the bolt stop pin holes over time. I've had a couple older 10/22's display that characteristic. Volquartsen and others came out with polyurethane bolt stop pin replacements, and they do wonders, both in reduced pounding, and also in keeping the crosshairs on target during prairie dog season. (Personal observation there...)
My Browning BAR and Auto-5 had bolt recoil buffers from the factory, as does my Colt AR-15. Go figure.
Some guns don't do well with buffers. The Shokbufs for 1911s used to disintegrate rather quickly, and during IPSC, I wouldn't use them at all - they caused malfunctions as they fell apart and left their remains in the guns. That soured me on the concept for that platform, so I don't use them in those guns anymore, either for fun or carry.
As for the AK, there's so much room in that receiver, and the bolt carrier and bolt's stroke is so long, there's no big problem having a 1/2" chunk of polyurethane riding the recoil spring guide's rear block. Even at full rearward stroke, the bolt is well aft of where it has to be to cock the hammer and pick up a fresh round from the magazine.
I bought my first BufferTech AK buffer to try in my brand-new milled receiver SLR-95, I think it was 1998. I was immediately impressed by the improved feel of the gun during recoil, with sight alignment being more consistent during firing, and no serious thunk as the bolt carrier impacted the rear receiver. That was all I needed to know when I bought my new stamped SAR-1, which was lighter and had a riveted rear trunion. I fired it a few times sans buffer, and then with it installed. The buffer has since stayed in that gun, and looks just fine after about 1K rounds to date.
I'm waiting for somebody to come loping along with the line that X-bazillion 3rd-world AKs don't have them installed. No kidding? And what happens when that 3rd-world AK owner wears out his Klack, hmmm? He contacts Ivan or Achmed and gets another one for relatively cheap, maybe a pig or chicken in trade. Try that here in BATF import-restriction country, where any AK may very well be the last one through the docks. We 'Merkins are lucky in that we can have semi-auto AK clones, but this ain't Somalia. I'll take care of what I have, thank you.