Have you seen how absolutely HUGE Morax and Asura are in the trailer? You don't often get any idea how big these demons can get unless you put them side by side with a human.
I’m sure it’s just for spectacle. I’m sure they blew half the budget on this anime sequence.
But…why are they fighting? They maybe weren’t best buds in the original scenario, but I don’t know why’d they’d come to blows. Eh, probably just the Rule of Cool > logic.