Devils are from another plane of existence. They are very rarely summoned to this world, and expect to get something out of being forcibly yanked from their home dimension. A contract, for example.
In fact, contracts are the only safe ways to deal with a Devil, and even then it's not completely safe because although powerful wards and spells of binding are used, few beings can phrase a contract to be watertight enough to keep a Devil bound for ever. Loopholes can and will be found even in the simplest of deals, even if it takes the Devil centuries, even millenia to accomplish this.
Devils are exceptionally proud and easily offended. This in combination with a vindictive nature and great powers make them perilous beings at best.
Still sometimes Devils must be summoned to undertake tasks nothing else can accomplish.
No one knows if there's any other way to appease them, although Zhe'raen certainly did try...