They're magically delicious!
Very early in patch 2.4's progressive development on the PTR, the ability to send raid icons over chat was implemented. The syntax is pretty simple, you place the name of the icon in those funny looking brackets, like so: {circle}, {star}, etc.
Personally, I'm not too terribly excited, it's not something you'll be typing on the fly, probably. I can't really see a Mage or Warlock(or anybody) taking the extra time to type that out properly when something goes wrong(ie death), as simple as it seems to do so. However, this could be a good tool for raid leaders, not so much the raiders. A raid leader could include these in a tanking/crowd control assignment macro. "/ra Matthew, tank {skull}. Christian, sheep {square}." That sort of thing.
Regardless of how often this will actually be used, it's a nice little feature, and I'm sure post-2.4 mods and addons will make good use of them, even if the players themselves don't.
