It's because mobile browsers have to deal with sites built only for desktops. So if they don't get this tag they wil emulate a desktop resolution but on a mobil

It's because mobile browsers have to deal with sites built only for desktops. So if they don't get this tag they wil emulate a desktop resolution but on a mobile screen. The tag sets the width to the devices actual width (although pixel scaling can change the display resolution) and the initial scale sets the scale to not imitate a desktop.