Charging the internal battery from an external one loses a lot of energy, battery charging is very far from 100% efficient A phone might not use enough energy for this to make much of a difference, but you need to build and carry beefier battery banks compared to internal batteries, and I’m not sure whether you’ll see a net benifit

Agreed for general use, but for people who just replace the entire phone instead of replacing the battery, it will reduce the waste of all the rest of the phone for some time.