The story definitely needs him. Also Sion needs him, and Sion knows it himself, no matter how much he also hates it. He's too good to do what needs to be done. I'm not sure yet whether Froaude has ambitions of his own or if he has simply concluded he can't be a figurehead himself and thus needs to rise alongside somebody more presentable. Although of course Sion isn't doing what he's doing simply to become a king of an even more powerful nation.