There's been a whole slew of negative feedback over Apple’s new battery case-- the thing has a hump that protrudes from the back of the device and doesn't allow the phone to lie flat. Arguably, it isn't the sexiest design Apple's developed; in fact, there's been a ton of press coverage criticizing the new case as ugly and expensive, so why did Apple choose to design its new case the way it did?
In the past, many of Apple’s designs were motivated by form as opposed to function, and often features were lost in an effort to make a device more appealing. In fact, products like the Macbook Air and iMac have gone to great lengths to look sexier, like losing their DVD drives in an effort to have a sleeker design--great for traveling, but not a huge plus when the product sits on a desk 99.9% of the time. Apple has even used smaller and thinner stress relief boots on many of their cables, making them look more appealing but having less durability. With the new battery case, Apple has chosen to go a different route: putting functionality over aesthetic.
The most obvious decision by Apple is that infamous hump, which is there to house the 1877 mAh battery. Some other battery case manufacturers make the entire case the thickness of the battery, but this is something Apple has avoided. With a handful of first party cases available, Apple wanted to ensure similar dimensionality to those existing cases. The hump lets the case have a similar width and thickness in areas where the phone would fit into a dock. If Apple had used a battery that was the entire height of the case, it would not have fit into docks.
One of the unique characteristics of Apple's Battery Case is its single piece design. Made of TPU (thermoplastic polyurethane), Apple's case flexes so the phone can fit inside rather than having a pull-apart shell like other current battery cases. Another major player in this field, Mophie, has over 80 patents related to its phone cases. There has been speculation from other technology and news websites that Apple's one-piece TPU design was to avoid Mophie's patents.
There's also been some criticism that Apple's battery case is smaller in capacity than other cases. Mophie's Juice Pack Air sells for about $99 (the same price as Apple's new battery pack), but has a capacity of 2750 mAH. The size of the battery Apple could fit into its case was dictated by the size of the hump--it's rated for 1877mAh while the internal iPhone 6s battery is rated for 1810mAh. Yes, it doubles the capacity of the iPhone's internal battery, but doesn't come close to Mophie's capacity. In response to that criticism, Apple said that its battery pack has iOS integration and won't impact the use of the phone (third-party cases have a tendency of interfering with the antennas and lightning port). In fact, the case even includes an additional antennae to help boost signal performance.
There's also been some criticism about Apple's choice to not include a power switch on the case, and that the case uses its battery before the phone's battery. From a consumer standpoint, this is beneficial feature, since the iPhone 6s does not have a user replaceable battery. Apple states, “ [The battery is] designed to retain up to 80% of its original capacity at 500 complete charge cycles,” so by relying on the battery in the case first, the charge cycles of the phone's battery will not be depleted.
Apple has been criticized for choosing aesthetic over function, but the new battery case is their first foray into function over form. It's a good idea, but not finding much love among customers. What is really required is a balance of both design and functionality: if Apple manages to reach that sweet spot, some of the criticisms could be put to rest.