This may be obvious to many, but if you are like me and do not trade-in your most recent device for a new one, you would probably have noticed the battery in your old device lasts a lot longer without a SIM card (which has been transferred to your new device) , which is now a WiFi-only device.
My old Z Flip 3 is a great WiFi-only device now that has a backup of all my apps and settings, should anything adverse happen to my new S25 and I need another phone quickly.
if everybody has budget like you then yes buy a new phone w/o trading in, it's not about backup phone, it's about we have different budget when buying a new phone
If I had to trade-in I'd trade in my S10e, not my Z Flip 3. I did trade in my S7 (instead of my S10e) when I got my Z Flip 3. I also do not get every new phone that is released, e.g. Z Flip 3 to S25 is quite a jump. Switching phones is a lot of work, I think. But sooner or later we all have to do it.
Using wifi doesn't require a sim card active or not i.e Sim card has service or doesn't not sure of a battery difference between the two but would think the phone looking for signal or service would be minimal
The cellular antenna/radio trying to get a signal (especially to pick up a weak signal) does use up quite a bit of battery. Maybe some of the apps know not to run routine things if there's no SIM card present too? e.g. can't send /receive SMS messages anymore. Also, screen brightness can use up a lot of battery.
