Lurkers can become brand/community evangelists even if they do not participate. If they enjoy your community enough to consistently lurk, they may spread the word, or recommend your community to others.
If you have banner ads on your site lurkers mean you have eyes on those ads, which equate to page views.
Lurkers are evidence that even if someone is not actively posting they are gaining benefit from your community. It’s the content that draws them in, but it’s the community and relationships that makes them stay. So if you haven’t got all, or some, of your community content open to public viewing you should. Give your readers something to draw them in, and that in turn will encourage them to register and post.
Image courtesy of Celenite