Deep links are a way to increase your PageRank.
When a search engine evaluates the links pointing to your site, the content the links target is taken into consideration during this evaluation. A link that is pointed at content pages buried deep in your site, rather than to a links page or your home page, is called a deep link. Links that go directly to a site's interior pages enhance usability because, unlike generic links, they specifically relate to users' goals.
Deep links are seen by the search engine as being a link to relevant and worthwhile content. When you are targeting links from another website, make sure that the links are targeted from your most relevant content page to their most relevant content page. Most high quality websites have links pointing at internal pages and not at a single, common page. The best way to create these internal pages is to point your links to relevant landing pages within your website.
Search engines identify common and deep links and rate them. The tendency is to assign more value to deep links. This is called deep link ratio and it is calculated by looking at the number of deep links and comparing them to links pointing at the website. You should encourage deep linking to your website, since a high deep link ratio typically tends to have a higher organic ranking, but a deep link ratio that is too high will seem forced and manipulated. A site with a seemingly manipulated deep link ratio will have a low rank.
The idea to keep in mind is that a deep link is better than no link at all, and may even be more profitable than a home page link, especially if your website has a bad navigational structure.