The Purpose Of Web Directories

In other words, It is lists web sites by category and subcategory that organizes the Web into major categories, including Business and Economy, Coputer and Internet, Socity, etc. Under each of these categories are a list of subcategories, and under each of those are another list, and another, and so on, moving from the more general to the more specific.
Simple example for it, To find out about SEO firm in web directory, select the Coputer and Internet category, under which you find a subcategory on the Internet Marketing  . Click on that and you find another list of subcategories like Website Promotion, several of which are pertinent to your search: PPC, SEM, SEO. Selecting any of these subcategories eventually takes you to Web pages so that visitor can easily find these sites by checking the category of their interest.

The Dmoz is the most widely distributed data base of internet content classified by humans.  Its editorial standards by best net-citizens on the internet.It powers the core directory services for the internet's largest and most popular search engines and portals, including Google, AOL, Lycos, HotBot, DirectHit, and others.

The other major purpose as below:
It cover many categories, so they may come up for a search query for numerous keywords or keyword phrases. Directories with a good page rank means quality directories and ones that are properly managed and organized have really great visibility to search engines.
By submitting your site into as many quality dir as you can, you are increasing your website link popularity, building site credibility and finally, your website SE ranking can improving.
Some of the more popular directories may provide direct and relevant traffic to your site.
Well indexed directory can provides a link back to your website. Generally,SE will be crawling these established sites more often the your new site. Once the SE sees a back link to your website, than the SE will crawls your website again.
Back links to your site appears in a respectable Web directory is good because first, it is a backlink and second, you increase your visibility for spiders, which in turn raises your chance to be indexed by them.

Submitting to directories are either give some direct traffic or a quality back links so that try to submit your website to more and more directory will build you website more backlinks,
improve ranking and a more traffic to your website.