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?
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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.