The comparison recalls the earlier analogy of sheep and a flock, to which lions were one of the threats (1 Samuel 17:34; Amos 3:12). A lion roars to establish its dominance, and so a lesser (male) lion would not dare to roar lest he invite a thrashing. When Peter, then, compares the devil to a roaring lion, he is telling his readers that the devil wishes to dominate you, and if you resist you may expect to receive his diabolic thrashing, to the point of his seeking to have you for supper. Resistance to the devil invites suffering. Knowing this fact is part of being sober-minded and being watchful.
8 Be sober-minded; be watchful. Your adversary the devil prowls around like a roaring lion, seeking someone to devour.