@f Oh, all the manufacturers started doing it back then. It was because they started including wireless charging in phones, but the back of the phone had to be something other than metal, which a lot of phones were back then.
They could've used plastic instead of glass too! It would've been cheaper and more durable to use plastic. However, Apple, Samsung, etc.. didn't want their phones to "feel cheap". And that's why we're stuck with glass.