I have noticed, that when I ask a question my flag icon below the post is enabled. I find this quite strange.
Why would anybody want to flag his/her own post?
I've recommended it a few times on a site I moderate on.
You can always see your own deleted posts - some moderators may suggest fixes, and once that's done, that's one way to get it undeleted.
You might also want to get a mod in to clean up comments to your post in bulk, or otherwise get moderator attention for janitorial work.
In a few rare cases, you might end up needing to request a redaction of some personal information that might have slipped in, or deleting a post you can't delete yourself.
So... there's a few reasons. You certainly would not need the full range of flags, but at the very least, you may end up finding the free form flag form is useful.
So.. many of the same reasons you would flag someone elses post, and a few more.