I have an Onyx Boox Max Lumi [1], there are 2 reasons why the "backlight" is very useful.
First off it's not really a backlight like you see in most LCD displays, it's a front-light, or edge-light that projects directly on to and is reflected by the e-ink display (the transparent top layer of the display acts as a wave guide for the edge lighting, and it's really quite an impressive engineering feat that they got it to apply so evenly on a 13.3" display area).
Obviously this will be useful when ambient lighting is too dim to even read paper, but the fact is current e-ink technology is just not as reflective, or high contrast as regular printed text on copy paper. The front-light illumination at low to mid levels (at least on the Onyx series) goes a long way towards making the screen background look almost as white as real paper. Granted if you are outside in the sun you will not notice the frontlight even at max brightness. But in dim indoor lighting, it's the difference between looking at greenish-grey kinda washed out image, and something that might fool a casual passer by into thinking it really is paper & ink.
First off it's not really a backlight like you see in most LCD displays, it's a front-light, or edge-light that projects directly on to and is reflected by the e-ink display (the transparent top layer of the display acts as a wave guide for the edge lighting, and it's really quite an impressive engineering feat that they got it to apply so evenly on a 13.3" display area).
Obviously this will be useful when ambient lighting is too dim to even read paper, but the fact is current e-ink technology is just not as reflective, or high contrast as regular printed text on copy paper. The front-light illumination at low to mid levels (at least on the Onyx series) goes a long way towards making the screen background look almost as white as real paper. Granted if you are outside in the sun you will not notice the frontlight even at max brightness. But in dim indoor lighting, it's the difference between looking at greenish-grey kinda washed out image, and something that might fool a casual passer by into thinking it really is paper & ink.
1: https://onyxboox.com/boox_maxlumi