Directories give back links to your site. These links help you rise in the search engines. There are also some niche directories that give you some traffic. Traffic from these directories also turn into more back links when people who have web sites like your site.
It will help you to generate back links for your website, also you can get deep links for your pages with deep link directories option. Some of very good directories can also sent you traffic on your website which can convert into sales. Overall it will help you gain more popularity over the Internet.
I guess this is a rhetoric question, I see that you're a member from 2007 in this DP forum , so I don't think that you don't know what directories are what for. I admit that value of a incoming directory backlink is smaller than a do-follow blog back-link for example... but still submitting to directories is an important part of SEO. As a bonus, some well established directories can also get you some traffic.
For people running a directory, it's really beneficial, as it's easy to build up high PR for your directory, then place links to your other pages. So for webmasters, directories aren't the best thing in the world, but if you want to start banking on SEO, buy a directory from a site like sitepoint marketplace, and you can start reaping the benefits.