Hiding it using CSS is part of the implementation of skip links, its only useful in browsers where css is not supported or is disabled. In that case the site navigation is typically a lot of content at the top and skiplink is an anchor to jump down the page past the navigation to get to the main content of the page.
See tutorial here:
http://diveintoaccessibility.org/day_11_skipping_over_navigation_links.html
and if you look at a site using the Lynx viewer, you can see it with no CSS, the idea is to have an anchor tag to jump past all the navigation links
If you go here:
http://www.delorie.com/web/lynxview.html
and view www.mojoportal.com you can see what it would look like in the Lynx browser which is a no css browser. Though you can't actually test the skip link itself unless you really use Lynx browser or something similar.
Joe