perhaps if someone wants to buy FSB-GB combo gun, because somewhere down the line they might turn it into SBR.
techincal reason behind FSB-GB combo is simple. it was created for rifles that were supposed to offer better muzzle velocity than AKS-U but still be somewhat more compact than full length AK (implemented in AK-102/104/105 series). It was achieved by reducing length of the barrel from standard 415mm (16.3") to 314mm (12.3"). At the same time this new 'carbine-version' of AK used same BCG and gastube as normal length AK - which was making it much more field-reliable and easy fixable unlike AKS-U that used proprietary parts that were not interchangeable with full-length AKs and therefore required to be returned to armory for fixing versus just picking up some repair kit from your TS officer or in the battle scenario taking appart KIA's rifle and swapping damaged parts.
off course here in US you can't have rifle with 12.3" barrel unless you go through SBR stamp bull shit. But nevertheless new combo gas-sight block 'made a splash' and became one of the signature points of AK-100 series of rifles, making it desirable feature to folks, who want to have newest and latest in AK development as oppose your average run of the mill 'ye-olde' AK-74 looks.
