I’ve always had a spare iPhone for the specific purpose of having a backup in case one of our iPhones is lost, stolen, or dies while it would be inconvenient to get a replacement - mostly for when we travel, but there are also times when it would be impossible to find time to get a replacement phone. I’ve never had it activated for cellular, and it’s only powered on to get iOS updates when those are released. It used to be that I kept my old phone as a spare when I upgraded, but last year I bought a 16e and gave my old 13 Pro to my son, whose old iPhone was in pretty rough shape.
Particularly when we travel, having a phone for us these days is pretty important. I do usually print itineraries, etc., as a backup, but almost everything is on iCloud (and the online service TripIt) and having a phone when moving around makes things so much easier.
So, depending on how much my carrier (Verizon) charges for this, I may very well use this feature, so that if something did happen to my old phone when I was traveling, I could quickly switch to the spare and be up and running with only minor inconveniences (for me this would be factory resetting my watch and pairing it with the spare phone.) And if something happened to my wife’s phone, or to one of my kids’ phones, I could turn off handoff, factory reset the spare, and let them activate the phone on their line until we were able to get a replacement.