Let's get this out of the way first; where the 28mm is refined and reliable, the completely new 56mm telephoto camera on the back of the iPhone 7 Plus is still an experiment in many ways.
Even from the first glance, you can tell that this is a lesser camera than the 28mm in everything but optical reach. It resolves less detail across the board, has a wider depth of field due to it's smaller aperture, and pales in low-light performance. And yet, it still handily makes up for all that.
Not from the extra zoom itself - though that has already come in handy several times - but for the completely different focal length and the artistic freedoms & challenges that allows for.
I've always been primarily a 35mm shooter, so shooting with the 56mm lens - which falls inbetween a true 'normal' and 'telephoto' focal length - is forcing me to experiment in new ways. I have to zoom less with my feet than I'm used to, the framing is completely different, there's the telephoto lens compression that I have to account for - it all comes into play.
For an amateur/enthusiast photographer like me this is extraordinary. Not only is there some handy extra reach when you need it, but there's twice the number of photographic possibilities with the inclusion of a second focal length built in.
Please don't think about it merely as 2x zoom, think of it like a whole second lens you're detaching & swapping onto your camera with only a tap of the screen. The creative options this affords - even with the merely okay image quality - are what make me love the iPhone 7 Plus's camera.