In some multiplayer servers, gold is either traded or used as currency. It is useful, as it has multiple tiers (81 nuggets = 9 ingots = 1 block).
Since Zombie Pigmen carry golden swords with them, it is likely that the gold nuggets are pieces from the destroyed swords, since they are not dropped upon death. Only 1/18th of the gold from a pigman's swords are salvaged (assuming 100% drop rate). As of 12w06a, the swords themselves are rarely dropped.
216 individual gold nuggets are needed to craft a full set of gold armor.
Iron Nuggets share the same shape as golden nuggets. The only difference is the direction they face.