The difference between flanges and valves?
Faucet valve is the same thing, or that the faucet is a valve in one of. That you can squeeze open, water or gas can flow out; can be turned off, the water not to flow.

Home tap is threaded screw on the pipe, but the factory with some of the tubes inside the water or gas, pressure is high, with screw connection was too strong, straight welding or in other ways to connect. Flange is one of a kind of connection method.