[rakoo]

If a package really is installed as a dependency, it will be marked as such. If for some reason you manually install a package that is needed by another one, then it's a good thing to mark it as dependency.

In the case of cleaning your system, it is interesting to mark an unused package as a dependency; the resolver (see https://www.archlinux.org/pacman/pacman.8.html#_remove_optio...) will see that it's installed as a dependency, but no package depends on it. It's a quick way to "tag" them as unused so they can be "garbage collected" at a later time.