I think battery pack chargers are more useful because they can charge a tablet, MP3 Player, some handheld game consoles, and other devices which charge by USB. Some bigger battery pack chargers can even charge a laptop like the Apple Macbook which uses USB-C to charge.
I can also re-use battery pack chargers on my new phone, and they seem more durable than internal batteries for smartphones.
Removing the back panel on phones with a removable battery is also kind of hard on some phones.
I think Battery pack chargers can charge some digital cameras and camcorders along with smartphones and tablets, so I think battery pack chargers can be useful if you have a lot of power outages, or you can't easily find a power outlet to use for charging when you are on longer trips, or in the forest, desert, or farm.
My phone doesn't have removable battery. At first it was worrying but later I got to know that... it's not that different and got more power cycles. I use a powerbank with me n case of emergencies.