Mods like Optifine or Continuity enable resourcepack creators to change the texture of a block depending on it's custom name. xali's Enhanced Vanilla for example, retextures barrels to have different variants depending on their name.
This does not work with multiplayer servers out of the box, because after re-joining or for a different player then the one placing the barrel, the server does not tell the client the custom name of that barrel. This mod fixes this problem. (Hurray!)