WP Core is indeed not part of wp-content or other subfolders…
ABSPATH, for a bunch of reasons should always go to the root of the site/domain. As far as I know, a whole lot of functions rely on that, too.
AdRotate needs to find the root path of the site. As far as I know the only reliable constant or function that does that is ABSPATH. I could use get_home_path() but often that doesn’t always work for some reason.
All other functions in WP only offer easy use of the content or plugins folder that I’m aware of. But what if you stick your banners folder in the root or something?
If you’ve moved WP core elsewhere, there are functions/constants for that – Symlinks often cause issues I’ve heard. As an example; I remember someone having a similar issue last year, They started out with symlinks, too. Ran into more issues changed to using the “official way” and didn’t set up constants correctly which made them think AdRotate had a bug, some trial and error later it turned out they did the constants wrong or something.