What it is for
At least on vowi.fsinf.at the cl_sortkey column of the page-table is highly inconsistent, sometimes including the namespace and sometimes not. A suspected cause is the frequent update through a variety of MediaWiki versions since 1.1 or so. I don't know which way is the correct way, in any case, it causes the sorting in categories to be broken.
Additionally, if cl_sortkey is supposed to include the namespace, it would cause a behavior we wouldn't like, since all pages are in just three namespaces and that would cause all articles to be sorted into just three letters.
This patch to CategoryPage.php fixes these problems.
What it changes
The page_title column is used for sorting instead of the cl_sortkey column.
Patches for different versions of mediawiki can be found at https://github.com/mathiasertl/MediaWiki-patches in the CategoryPage.php folder.