Directories are one way of getting quality inbound links to your website. It's not a good source of direct traffic as you said. Submitting your site to good and high-PR directories is one way of getting quality links that are highly valued by the search engines. Web directories are mainly human edited, thus a website must pass a strict criteria to get listed, hence quality links are only listed. These links are search engine friendly. Since they are organized in highly focused categories making them an invaluable resource for search engines to measure the quality and the relevance of a website.