It's now possible to carry a spare battery or two, instead of lugging a portable power bank and slowly charging your phone. This is great news for outdoorsy types, travel, long bicycle rides, hiking, and so on.

but they require you to have a special charger, or typically you have to charge them one by one from your phone. Swapping batteries also requires downtime (not easy mid-flight, cycle, or whatever you're doing). A portable battery charger is much better imo. Plugging in a cord is always going to be easier and require less hands and focus than replacing a battery and keeping any kind of dust-and waterproofness.