The Black Heart of a devil not only represents its number of lives for the vessel and determines the number of times it could die but it is also the source of their power. With each Black Heart destroyed from a devil, their power decreases by a significant amount. The fact that Beelzebub sacrificed one for Ark means that he wants to help them sincerely.
Adrian wanted to ask something to the Fallen about Ark being the prophesized king or something but that would be for later. The only thing that he needs to be concerned about is Ark hatching from his egg. Adrian grabbed the Black Heart and hovered it in front of the Nephilim Egg.
"Since the devil actually offered something of his body, I think we should do the same, but I will be the one to sacrifice." Rikbiel stated.