This should be used with all gas line maintenance or installation to be absolutely sure you don't have leaks in fittings. Hooking up a new water heater or stove? Replacing a regulator? Every connection should be checked.
Detects leaks in anything that can be tested for air or gas pressure. Just spray on the connection after you have finished and look for bubbles.
Keep it in your toolbox for those days you think you smell gas.