During each of your opponent's end steps, that player has to choose two creatures he or she control, then sacrifice the rest. Against aggressive strategies, this is absolutely devastating. While not every deck focuses on having lots of creatures out, the Archfiend can turn up whenever an opponent is trying to win by just turning a bunch of creatures sideways.
There are quite a few decks that rely on creating lots of tokens or casting tons small creatures in order to simply overwhelm opponents. But the Archfiend can absolutely punish these strategies. With Fate Reforged introducing the Mainfest mechanic and Elves becoming more of a force due to cards like Collected Company and Shaman of the Pack, the Archfiend may be dropping in a lot more often.
In Commander, the Archfiend of Depravity is a killer. Not only are Demons a popular tribe, but Black decks love things to die. The Archfiend makes more things die., and that effect is extremely powerful in a multi-player setting. Having a 5/4 body is cool, but that means it dies to things like Stoke the Flames and Languish in Constructed. Still, the Archfiend only has to survive one turn with your opponent having to sacrifice a bunch of creatures to it for it to be worth casting. Best of all, the Archfiend never hurts you at all.
While Archfiend of Depravity may never be a maindeck play outside of casual "kitchen table" Magic and Commander, you'll probably be seeing a lot more Archfiends in play if aggressive creature based strategies become more prevalent in Constructed. Dragonlords have no problem with him, but Elves, White Weenie and other decks that depend on their board states being filled by armies are going to be sad if the Archfiend of Depravity says hello.