I put one on (after I scratched the screen with the Apple Pencil). The only thing I can figure is that some grinder dust got on screen and pencil tip found it and scratched glass. Found protector on Amazon....much cheaper than Apple Store.
Yes, I put one on just to see if it would give me more of a "paper-like" feel when drawing. It does to some extent. Is it really necessary? No, but I kept it on just because I got used to the feel. The brand I bought was Tech Armor and a 2 pack was inexpensive from Amazon.
I have put on a screen protector, but not the glass ones but a sticker one. Yes Apple pen makes scratches on the screen. If you do a Google search you will find that it is the Apple Pen tip itself that makes the scratches.