Are wordpress posts stored in the SQL Database?
Yes.
...then what is the point of the XML file that you can import/ export?
Pages, post and/or comments separate from everything else in the database.
If I export the XML file and change it then re-import it - then does that also change within the SQL Database?
No, importing something does not synchronize it with itself if it is already there. So no, an import will not change anything, it will merely add something new even if that means you end up with duplicates.