Directory submission is helpful for increasing PR and SERP. Even all directories do not help in ranking because some are automated approve directories. High PR, Do follow and fast or regular caching directories are more helpful.Suppose, your site is approved by a high directory and it is fast caching by Google and other major search engines as well and the page and category where your site is submitted is also cached then it will notify to google for your site.You will need to put website URL, name, about, sometimes keywords,meta tags.Same thing happens with at least 50 web directories means 50 times google bot know about your sites and such it helps to improve your ranking and PR because 50 web directories are giving your reference to google, now your site is well know for google as well.

If you are serious about increasing your website's visibility and increase traffic ratio, it is important to have inbound links pointing to your website from high ranking websites. It is better to have inbound links from a page rank 4 website than a rank 1 website. So each and every link from a page rank 5 website will actually count a lot more than links from page rank 1 websites.