People replace factory gas caps for a few reasons. Some factory gas caps rattle and this can be quite annoying, so people replace them. Another reason is because of poor design. A lot of riders wear gloves when they ride. Trying to open a factory gas cap with a smooth surface does not allow for much grip. Spinner gas caps and other designs are easier to grab with gloves on. Another reason is for security. Some people want to have a lock on their gas cap. This can help eliminate the risk of people stealing your fuel, or putting something into your tank. The most common reason is simply aesthetics. Pop-up gas caps are popular because they sit flush on the tank, and some people think they look cool. Sometimes a rider simply wants a different material or finish gas cap, such as a black or brass gas cap for Harleys. Changing out a stock gas cap for a custom one is a very quick and inexpensive process, and can add some personal style to a motorcycle.