Reversibility is the clearest benefit to the end user. Also it's a much more durable connector than most, the cable end is just a flat piece of metal as opposed to having pins and whatnot.
The stress of holding up the entire mass of the connected device (that can be as large as an iPad Pro!) is on the connector; and the stress of being held up by that connector is on the tiny part of the device that grips it. That'd be impossible with a USB connector (even USB-C), on both sides.
Apple's logic: the Lightning jack only appears on accessories; while the USB-C jack only appears on "computers." There's no Lightning-to-Lightning cable, because it makes no sense to connect one "accessory" to another. All "accessories" come with a Lightning cable, which is USB on the host end for now (because most people don't have computers with USB-C yet!) but will likely be USB-C on the host end in the future (and then you'll need to buy a reverse adapter—female USB-C to male USB—to connect it to old devices. Those will probably be common enough across brands once the USB-C-only accessory market picks up.)
All that being said, it'a interesting to use this logic to deduce what Apple does or doesn't consider an "accessory." The iPhone, iPad, iPod? Accessories. The Apple TV? Computer! The port to plug it into a computer—for XCode provisioning et al—is USB-C! To connect to it right now, you need either a MacBook with its charger cable; or the new MBP, plus an extra C:C cable; or a USB-A male:male cable (rare!) plus Apple's "MacBook USB" (female-USB:male-USB-C) adapter. A male-A:male-C adapter would be great, but nobody has yet made one. The market is very nascent.)
> if you have a brand new Macbook Pro and iPhone 7 you can't connect the two without an adapter
Without a cable, not without an adapter. Very big difference. I can use the same cable to charge my phone from my MacBook's power supply and from the MacBook itself. When I travel, I only take one power supply and two cables. Even if the iPhone and the MacBook used the same connector, I'd still take two cables so I could simultaneously charge them both, so I don't have to carry any extra hardware.
Yes, it would be nice if they could use the same cable, but at least I don't have to take any extra cable or dongle for charging my iPhone (I still need dongles for other things, though).
Except the cable in question is bought separately. Dongle, cable, adapter, whatever you want to call it: It's two products from the same company that has a reputation for an ecosystem that works extremely well with itself, and if I walk out with two boxes, one Macbook and one iPhone, they cannot be connected without a trip back. That's absurd.
