Short part of the answer is:

If you submit your site to alot of directories it will improve your SERPs in the search engines.

Link populariy is a very big part of SEO. Using web directories is a great way to build more links and increase your site’s SERPs. People will not use web directories alot but spiders are. Spiders will still regularly index them and the links they contain. This is why submitting your site to web directories is still very usefull.

If you want to read more go here:
Why You Should Use Web Directories !