When does the "merge this plant entry with another" come into play?

That's if you have duplicate plants and one of them needs to be deleted. Admins can merge the two together which automatically moves over all the images, comments, etc, and deletes the now-empty one.

The plant admins often decline proposals to merge plants and then handle it ourselves. Sometimes because data, which isn't merged, will be lost or other reasons,... We often need to prepare the entry that will remain. Proposing a merge is helpful though.