I had to replace batteries on (2) iPhones recently and took them to the Apple store to have it done. 1) They use real Apple batteries, 2) have the proper tools, the cost was only $49. Sure I could take it to a kiosk in the mall or a cellphone repair shop but what kind of garbage battery will you get ?
That $49 also covers a 90 day warranty on the work. If something goes wrong, they'll likely just replace it with a white-box refurb unit, which will have a new case and new battery. For some users, it will be in better condition than the phone they brought in for repair.
I'm not certain whether that's something AASPs like Best Buy can do, but an indie shop or the mall kiosk most definitely cannot.
Unlike cars, most of which will never grace a dealer's shop ever again once out of warranty, there are many advantages to having Apple do the repair, under most circumstances.