It protects your wheel spokes if your chain drops off the biggest cog into the gap between the spokes and the cogset. If you are climbing a hill with a lot of pressure on the pedals, chain drops off, and you really jam your chain in there, you can cause a good amount of damage to the wheel. Prob. called a "spoke protector" in a parts catalog. Just keep it there and forget about it. If you get to the Tour of Switzerland, have your chief mechanic double check your limit screws and take it off.