Those are triggered for before the fold images, I primarily use them for above the fold content to try and get as close to immediate loading for the user. That way they can interact with the site quickly if they aren't interested in the image - maybe click a menu item, etc.
In the third world we have to be especially sensitive to these things because mobile data is often incredibly slow, even on 4G.
I do not like it when people use these images. I think you get a better result when you use the now supported loading=lazy attribute on images https://web.dev/browser-level-image-lazy-loading/