Not everyone lives somewhere with natural gas service, and gas service could also go out in some types of disasters.
It’s possible to get larger propane tanks and refill them more cheaply (rather than exchanging expensive grill tanks at the store). Some hardware stores will refill tanks directly. You can even get very large tanks (refilled by a truck that comes to your house) if you want weeks of power on tap. But this is probably overkill for most, unless you’re very remote.
I tried that but services want regular deliveries and aren't interested in refilling emergency tanks. And a tank large enough for decent power backup will be hard to move.
Ah, ok. They make auto-change regulators that allow you to hook up two tanks, and two 30-60lb cylinders should last quite a while. But if you have natural gas service and aren’t in an earthquake or hurricane zone then that’s probably better.