So essentially, if you own the tools already, replacing your battery is $20 cheaper than taking your device in, or mailing it in. If you don't already have the tools, the cost of parts plus the tool kit rental means a repair cost significantly higher than just having Apple do it.
Plus, you then get to assume ALL of the risk when something goes wrong during the repair.
When we were a repair center, there were plenty of times when things would go wrong, such as a screen cracking when taking the device apart, ribbon cables getting damaged during the repair, etc. And this is with experienced technicians.
It's cool that this option exists now, but people are going to freak out when things don't go as planned, that's for sure!
The economics of being an authorized service center have gotten pretty crazy. As Apple stated, the price they are charging to end users is the SAME price they charge their authorized repair centers. ie, zero margin available for the repair shop, and $20 of room to charge for labor, plus they get to deal with all of the risk of doing the repair.
It's no wonder that legit phone repair shops are flash in the pan younger kids that think it's a great opportunity until reality kicks in.