The paperwhite with backlight on is only for reading in a situation with dim lighting.
Which is has always been an eye-strainer, even with conventional books.
At least with the backlight, you have the option to increase lighting a bit. Not as comfortable as natural lighting, but better than reading a book with little light.
However, when there is ambient light, the backlighting of the paperwhite adds almost nothing.It looks exactly the same on a sunny day outside, sitting in the shades, with backlight on 100% and 0%. It looks like a piece of paper, readable, but not glowing.
So it is not meant to be used with a backlight.
but you have the option to do so.