Because of the enormous amount of attention paid to the visual design of Apple's new Smart Battery Case for iPhone 6s, a lot about its functional design has been overlooked. Including by yours truly. Though the actual battery inside the case is smaller than many other, similar cases, the "smarts" Apple built into it the battery case means it's also much more efficient.
The coupled passive antenna helps maintain better signal. The better the signal, the less power the radio draws. (If you've ever seen your signal drop and your iPhone get really hot when in a case, you're basically throwing away the extra charge.)
Your iPhone knows when it's connected to the case rather than a plug so it's smart enough not to start any backups, networking, background tasks, "always listening" Siri on iPhone 6, or other processes that cause excessive drain. In other words, your iPhone stays as power efficient in the case as it does out.
If you plug the case in, though, it's also smart enough to open up all those power-hungry processes.
The case prioritizes keeping your iPhone charged at all times, so you never have to worry about whether or not you have the best power level possible, or that you're unnecessarily racing through your battery's charging cycles. You just put it on and use it as needed, and it just works.
Both the lock screen when you first plug into the case, and notification center whenever you pull it down, tell your precisely how much charge is left in your iPhone and in your case, so you make informed decisions about how much you use them and when.
There are some other benefits as well, including the one-piece design, and some drawbacks, including the narrow headphone jack. See my Smart Battery Case for iPhone review for more.
Not all of the power-saving points will be ideal for every iPhone owner, either.
Some people might want full-on backup, background, and networking activity when using a case. It might be exactly why they bought a case.
Likewise, some might want to be able to turn the case on or off independently whenever they like and for any reason they like.
Cases that allow for exactly those things are, of course, already abundantly available. The Smart Battery case tries to solve the same problem in a different way—admittedly a way only available to Apple.
While big batteries look impressive, the weight and bulk comes with a usability cost. If they're too heavy or too bulky—and every atom makes a difference—you may stop using them or stop using your iPhones while they're in them.
By keeping the Smart Battery Case for iPhone 6s light, and making it smart, Apple's offering an alternative for people who just want some extra battery life. Not all the time, but for when they'll be using their iPhones more heavily than usual or for when they'll be away from power for longer than usual.
If you hate the case, you can absolutely keep hating it. If you hate the people who don't hate the case, I guess you can keep doing that too. But, once you understand the functionality, like with the design, at least you can hate smarter.