APNG is the format Apple is using for sticker packs in iMessage. I'm gonna go out on a limb and guess Google is ramping up to announce a sticker store for Allo... sticker pack devs could port them over without any effort.
Small pictures to put in chat clients, rather like large emoji. It seems pointless, but I've had friends complain about going from clients that support stickers back to text chats like irc.