This page lets you register your site at tikiwiki.org, that means your site is added to the category tiki sites in the directory on tikiwiki.org. All you gain is an additional link to your site, no more, no less.
What we (the tiki developers) gain is an overview who is using Tiki for what. This may help us to decide which features to develop / enhance next.
screenshot of tiki_register_site.php: