Useful in crafting, as they reroll all modifiers (and the number of modifiers) on a rare item. This makes for unpredictable results, so getting a specific modifier is not easy. Using multiple Chaos Orbs on a piece of equipment in the hopes of getting at least a few good modifiers is called ‘Chaos spamming’.
These orbs also play an important role in trading. They are the “silver standard” in the player-driven economy and are often used to buy low to mid tier items.