A web directory categorizes websites so that a person can manually find information they are looking for, rather like a phone book. Directories provide an alternative to search engines because rather than just the big sites being listed at the top, it is the user who chooses which site to visit by reading a description of the different sites to see which matches their requirements best. Obviously the more directories you can submit your website to, the more traffic your website will receive.
Submitting to web directories will therefore have two massive benefits for your website: the first is that your website will get direct, targeted traffic from the directories themselves, secondly your website will obtain a good quality, one-way link from every directory you submit to, this will drastically improve your search engine rankings and Google PageRank, often moving your website many pages in the search engine results.