The most attractive physical attribute of an iPhone is the sleekness of it. When iPhone 6 was first launched, it garnered much criticism for the plastic lines that you can see on the flipside of the phone. The smooth, sleek surface was gone and many wondered if Steve Jobs would've agreed to such an idea. But ever wondered what those plastic lines are?
They are antennas! In the older models, they were hidden inside. But the metallic body of iPhone 6 meant that the radio waves would not be able to escape the phone, hence the plastic lines.
Some believe that the plastic lines are also a result of customer complaints of poor signal strength in the previous models, and this was the company's attempt at extending the antenna length.
If you too are worried about the lack of finesse in this model, then calm your nerves. iPhone 7 tries to handle the problem by trying to replace the antennas with a metal that will allow radio waves to pass while retaining the strength of the metal.
Another mystery solved. You're welcome!