The pyrophoric qualities lend themselves well to igniting ammo racks, but this quality is secondary to DU's density and fractal patterns. When it strikes a hard surface, it doesn't just ignite, it fractures in such a way that the tip of the penetrator remains sharp. Remember, its flammability doesn't matter much if the fires it starts are outside of the target's armor; it's when fire gets inside the tank that things get spicy.