This is a special tool to remove the valve core of Stan's Notubes, which cannot be removed when talking about tubeless tires. It is compatible with French valves (French style) and American valves (American style).
It is used to remove the valve core in order to inject sealant using an injector. Also, when the valve core is removed and air is injected, air can be fed into the tire at once, making it easier to inflate the tire.