If you use images for your main navigation it’s especially important to offer a duplicate set of navigation links in your footer and, even if you use text links at the top, the duplication is still helpful. You want to make it as easy as possible for people to find the content they are looking for on your site.
Often times the footer will link to additional information such as terms of service as well. Things that should be easy to find, but not at the expense of taking up real estate on the primary navigation area of the site.
