@edit1754,
I think there are two reason for the glass layers:
1. This enables the manufacturer to use the same panel for the non-touch and the touch option. Basically, this is the matte non-Touch panel, with the glass layer and touch digitizer added on top.
A "On Cell" touchscreen, like the one you are describing for the IdeaPad 5 Touch, needs to be designed for touch from the start. Obviously, not every panel made is a touchscreen, so the choice of panels the manufacturers can choose is more limited. The glass layers enables them to offer any panel they want with touch.
2. This devices supports a digitizer pen, and the glass layer protects the display when using the pen. A pen on a matte On-Cell touchscreen would probably damage the screen panel.