We started using chainring covers in 2005. They started out as flimsy plastic, and are now made from steel, so they look nice. They essentially have two uses. Firstly they will help keep grease off of your pants, or leg ( if riding without pants ). This is a nice added layer of protection. Additionally, it helps keep the chain on the chainring. Keep in mind that if everything is in order the chain can never fall off the chainring. However, if the rear derailleur hanger is bent, it will create a angular force that may pull the chain off. In other words if your chain is slapping around, please check that your hanger is in a parallel plane with the cassette.