Why submitting site to web directories

Search engine's crawlers (spiders) are looking for new sites to index in database and better way to do it through submit links in various directories. So that submitting website in directory is help to get your website easily indexed.
Getting incoming links to your site is the important ranking factor in the major search engine ranking algorithms. Directory submission helps you to get many of back links so that it can increase link popularity of your site.

You can get one way back links through submitting site to web directories and inbound links to your site is one of ranking factory for major search engine like Google, Bing and Yahoo.so, link from quality directory can help to improvement in search engine rankings.
You can using your main keywords or keyword phrases in title of directory listing and Most directories are use title to pointing back to your site.
Most directories allow you to provide a link with keywords describing your site. It’s not easy getting links to your site with the keywords you want and directories serve as the easiest medium for getting such links. Getting links with targeted keywords can have search engines favoring your site over others when ranking for the given terms. Using varied link anchor text you can increase the possibility of your website being found under these keywords or phrases when they are typed into a search engine. After all, if you want your website to be successful, it would have to be found by people and the best way for that would be your site coming up on top for the searches that matter. When people search using particular words, you would want your site showing up in the first few result for those words. Most directories are free to submit to given the benefits, it makes business sense to having your site listed in as many SEO-friendly directories as possible. After all, you do not lose anything for doing so.
You can get direct traffic by submit your site to high quality directories like dmoz.org, joent, yahoo directory etc.