The Blood War
The Blood War, a savage quest for annihilation that ravages the Lower Planes, is what keeps most of the fiends busy most of the time. It's supposed to be a war of tanar'ri versus baatezu, Abyss versus Baator, but everyone and everything down there seems to be involved. And neither of those "sides" can be trusted, either. A baatezu'll turn stag on his brothers if it'll gain even a little power out of the process, and the tanar'ri have always been at odds with eachother. In between them are all the gehreleths and the yugoloths, eager to fight for the best-paying side. It's a war that's been going on as long as mortals have lived on the planes and it's a war neither side has yet to win.
Nobody even knows what the exact goals of the war are. Genocide's the best guess - the tanar'ri and baatezu'll fight each other until there's none left. Why? What berk understands a fiend's mind, can figure out its depraved depths? Maybe the local powers are playing games and making them fight. Maybe they fight just because they want to. Nobody knows for sure how the war started, but a lot of folks figure the Lady of Pain had something to do with it. Some barmy philosophers claim that she's one of them, a fiend now turned stag against her own kind. Maybe that's true, and maybe it ain't.
The only thing known for sure is that the Blood War's a grim blessing to the rest of the planes. Every big move by one force triggers a reaction from the other side, since both groups are desperately afraid of the other gaining significantly more power. Sure, it makes travelling the Lower Planes dangerous - especially the battlefields of Gehenna and Carceri - but it keeps the fiends busy.
The Blood War creates a great need for supplies, weapons, troops, and leaders. Many bashers capitalize on these needs to earn some jink of their own. Some brave (or barmy) planewalkers deal in weapons or mercenaries with either or both sides. Those who don't end up in the dead-book often come out of it very rich. The yugoloths devote a great deal of time and energy in helping both sides - but only so they can earn a fair profit.