Which component is necessary for gas appliances to ensure proper installation and operation?

The manual shutoff valve is essential for gas appliances as it provides a critical safety mechanism. This valve allows for the immediate cessation of gas flow to the appliance in case of an emergency, maintenance, or malfunction. It enables users and technicians to control the gas supply, ensuring safety during installation and servicing.

Having a manual shutoff valve is not just a matter of convenience; it's a fundamental requirement in gas appliance setups to prevent leaks and potential hazards. While other components like electrical connections and pressure regulators contribute to the overall functionality and efficiency of gas appliances, they do not serve the same direct safety purpose as the manual shutoff valve. Therefore, the presence of a manual shutoff valve is a necessary feature for proper installation and safe operation of gas appliances.