I don't think I know anyone who owns such a device or carries around a spare battery these days. Up to about 8 years ago, I had an external charger for my cell phone batteries. The original Motorola flip phone I used had chargers that only charged the battery while it is not attached to the phone. My LG phone had a charger that you can stick the phone in or a spare battery. Since then, all phones are charged by connecting the charger cable right into the phone.
Nowadays, the only extra charging accessory I use is a car charger. I find that is good enough. I also find that it is easy to find a charger almost anywhere I go as micro-USB chargers are pretty well standard everywhere. I don't recall every feeling I would like to have a spare battery with me like I did 10-15 years ago.
Another reason why I used spare batteries a decade ago and not now is that back then, battery technology isn't as advanced. Fifteen years ago, the standard batteries are nickel-metal hydride. They had a bad property where their lifespan shortens if you do not wait for them to be fully discharged before charging. I would keep the battery in the phone until it dies and then swap and charge my old battery. Usually it takes days before the battery runs out of charge.
Modern phone batteries do not need to be discharged before you charge them. I would just charge my phone every night regardless of how much or little charge it has remaining. A full charge will usually last a whole day for me. On those rare occasions where I need to charge my phone part way through the day, I can always do so in the car or somewhere else. A spare battery and extra charger just doesn't provide enough of a benefit for me at least these days.