There are some technologies that have been used in the past that are no longer considered efficient or safe and therefore fallen out of use. However, since some appliances have fairly long life-spans, it is possible that you have an appliance that is still using outdated methods but still functioning.
Take pilot lights for example. In the past, this was the main way of lighting a gas-powered appliance. A pilot light consists of a small flame that is fed by a constant supply of gas. This flame is then used to light up the appliance when it is in use. The problem with pilot lights is that they can go out, leaving the gas to escape into your home. While there should be a safety shutoff, this might not work on an older appliance.
If you ever need to re-light an extinguished pilot light, you should first make sure to check whether there is gas in the area or not. If you are not certain, you should air out the area first.