I recently bought a cheap tablet with a tab that allowed you to remove the watermark layer of the screen protector on first use. The tab was so glued to the protector that I ended up peeling away the whole screen protector thinking it was part of the top layer meant to be discarded. With the screen unprotected and completely exposed I couldn't believe how hard it was for my eyes to keep focus on any part of the screen image not even for 1 second. The image is just too sharp and my eyes don't like it. I was thinking oh great I just bought another tablet with crappy screen destined for the junk drawer. When I put the protector back on again sometime later, I was amazed by the sudden transformation. My eyes could tolerate the screen display again. The way to explain it is the plastic softens the screen image before the light days reach your eyes. It acts as a sort of natural soft filter like this image below (left=no protector, right=image softened by protector): $this->handle_bbcode_img_match('https://www.howtogeek.com/wp-content/uploads/2017/07/img_5964472333658.jpg')
Another dangerous misconception I was having was that if I ever bought a cheap tablet meant to be used for a couple of years then thrown away, I might as well start using without the screen protector to experience a crispier image quality not caring for any scratches to the screen. This is simply wrong! The image looks so better WITH the screen protector.
Lastly, another area where I experience eye discomfort is computer monitor screens. The old days when there were CRT monitor, the phosphor dots that make up the pixels were naturally soft and round. Modern LCDs have pixels so square and crisp they appear jarring to my eyes. I'm gonna try to apply a large screen protector to part of the monitor screen at the places my eyes like to rest upon to see if that too will act like a natural image soften filter to make it easier on my eyes...