The Blood War

Though the average citizen of The Material Plane may have trouble differentiating between devils and demons, scholars and adventurers with the misfortune to meet them know they detest each other. Though they both represent primal evil, the Devils are an incarnate of law while the Demons are the incarnation of chaos. Thus, they will always fight each other, be it passive scuffles after a coincidental meeting or the great, everlasting siege of Avernus, the first layer of The 9 Hells. This everlasting conflict is known as the Blood War, and takes place all over the multiverse at all points of time. Recruitment for the War (and for power's sake) is typically the main motivation of demons and devils on The Material Plane.
The intesity of the conflict and the exact location of the front lines vary drastically from moment to moment, a balance remains - never can one side truly gain the advantage, lest they win the war and set their sight on the rest of the multiverse.