The most important thing for anyone who has a website is visitors. Without visitors, your website may as well not even exist. In order to get visitors, you can spend a lot of money on an advertising campaign or build a large amount of quality, one-way links to your website to better your search engine rank.
Google, Yahoo, Bing and other search engines rank sites by how many other sites link to them. So the more links you have to your website, the higher your search engine rank should be. By submitting your website to many different quality directories, you increase the amount of one-way links to your website and thereby increase your search engine rank.