I can only guess but I would think for stability or production runs. As illustrated by old mossy, a forend nut adaptor would get you to the same place and possibly for cheaper. I would go the cheaper route to acheive the same results. You'd have to shop around because pricing looks about even. With the adaptor option, you won't have a spare slide sitting around or maybe you could sell it for not a lotta dough. If you sell the shorter one then you limit your options later. I think I would go with the adaptor so you can use what ever fore-end you wanted.
Either way good luck.