There has to be enough fuel in the fuel bowl to keep the engine supplied. The float and needle valve control how much fuel enters the carburetor. As the float falls with the consumption of fuel, it relieves pressure on the needle valve, allowing more fuel into the bowl. The float rises against the needle valve, closing off the flow of fuel to the bowl. In normal operation, float movement isn't even perceptible. It maintains a smooth flow of fuel into the bowl.