I will never understand the whole lets shine a light at the display, people put on top of their monitors

to me, it was helpful for night sessions, as I didn't have good light that would be too strong. This one does not shine into screen, so it's good for the eyes. But certainly not a must or anything you need.

Isn't it better to have light on the back of the screen? Like illuminating the wall slightly?

maybe, but then I don't see my keys or stuff on the table. It dims automatically and has different color temperatures. It's practical, I don't have space behind my desk, and it's not that expensive. I didn't thought too much about it actually, when I bought it :)