Apple never keeps even a dot on its hardware if it doesn't have a function. But why does the iPhone 6 have this Gap/Thick line on its edge? Any clue?
Those are for the phones antennae (theres one towards the bottom of the phone as well).
The anodised aluminium causes really bad signal attenuation, hence the plastic-like strips are necessary to create a break in the metal shell.
I decided to delve a bit further and found that Apple is trying to get rid of them.
The reason is an antennae for the phone, since having a smooth iPhone would cause horrible and impossible coverage(quite literally). Since the iPhone has a lot of bands to cover as many network as possible, the "split" the metal back, and many of the strips on the device caused by this are the antenna.