Nice
I never had an iOS 7-cabable device but whenever I see the parallax on someone else's device, I find it extremely pleasing. It not only looks nice, it feels nice. There's depth to it and to me, it's more than just a gimmick.
The opposite of flat? Maybe.
Included as an option to offset the concerns of people who found iOS 7 too flat? That's a wild guess.
Technically it may be completely unnecessary, and I don't know whether it's a USP, but parallax was probably #3 on the shortlist of things that were long term attractors towards my first ever spend of a significant chunk of disposable income on a lovely chunk of Apple hardware.
Numbers #1 and #2 were CalDAV and CardDAV.
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(Off-topic: as things turned out, my plans to purchase an iPhone – probably 5.x hardware at the time – were wiped out by the detraction of Yosemite. I continue to enjoy a refurbished first generation iPhone … I am, unapologetically, one of those people who makes things last, who expects things to last, who does not rush to purchase a novelty simply because it's novel.)