The main point of it is for moving your posts somewhere else, or from somewhere else.
For example, you can export your Blogger blog (they have a similar tool), and import them into your WP site. Or the other way around, export from your WP, and then into Blogger, or many of the other blogging platforms.
You can also use is to export from one WP site, and import them into a new WP site. Like from your local installation (when you are developing the site), and then import them into the hosted/production site.
I haven't tried exporting/importing from the same blog. I would think you'd get an error that the post already exists, or you'll end up with duplicates.