The answer to the OPs question is another question: How comfortable would you if you lost your one & only iPhone? (Or if it suddenly died). No big deal? Devastating?
If an iPhone is an important part of your life that you rely on - then having a backup in the form of some older cheaper model seems like a good idea.
I kept my old iPhone 5 for this purpose. It's ready to go as a backup if disaster strikes.
Having a working backup on hand is essential, even if one has AppleCare+, as the problem phone may need to be sent off for repair.My iPhone X is my daily driver. I have an SE which I have as a back up in the event my X takes a catastrophic drop which renders the device inoperable. I have AC+ on my X, but I like having the SE just in case.
I also have a Galaxy S8 for when I want to use Android OS. But that device is really only used on WiFi.
If something happens it’s easy to get a new sim and deal with a replacement later.As the title avobe ^^ For what purpose do keep a secondary iPhone?
Having a working backup on hand is essential, even if one has AppleCare+, as the problem phone may need to be sent off for repair.