The valve core is a critical component of your tires. It's a small metal spring-loaded device that fits inside the stem; it allows air in when you attach a chuck to inflate the tire and prevents air from escaping during normal operation. Removing/installing the core without the proper tool can easily damage the core and or stem inner-threads and prevent proper functioning - which means you either can't get air into the tire or the air leaks out. This tool is the one you'll see professional mechanics and tire technicians use to do the job right. Universal fit on all Schrader-type valves, which are standard on almost all air-filled tires on cars, trucks, trailers, ATVs, motorcycles, tractors, lawn/garden equipment, carts and most bikes (some road bikes with skinny tires use a taller & thinner valve called a Presta valve). Sold in sets of two identical tools because once friends, neighbors or co-workers see you use one, they tend to get permanently 'borrowed'.