Reason was that I just wanted `<office:styles>` and `<office:automatic-styles>` element trees, and not have to deal with any other content in ODF documents. So this code did not need me to think about how to compile correct complete ODT document content and then still extract only the two subtrees again.
So no real hard reason, just quicker coded and seemed to reduce things to minimum needed for subject on test, without adding too much overhead.