Which kind of directories are you talking about - website directories, article directories, image directories......? Whatever the case, I think the biggest advantage of being listed in a directory is to have a backlink to your website. The more popular your presence in the directory, the more chances you have of getting traffic to your site.
The importance of a directory is to boost your SERP rankings by getting a backlink. In addition, if the directory has traffic you will be able to get visitors from the directory as well.
Niche directories will help your site with a little bit of traffic, but will help you most by helping your site rank for it's keywords. General directories probably won't give you much traffic, but they help a little bit in the SERPs.
For me, mostly bloggers and webmasters send their blog and sites respectively to literally hundreds of directories. I think that is pointless after a while since the number of directories with quality PR is limited. These are the top free online directories: http://www.dmoz.org 9 http://www.lii.org/pub/htdocs/home.htm 8 http://www.femina.com 7 http://www.gogreece.com 7 http://www.sacentral.sa.gov.au/site/page.cfm 7 http://www.worldhot.com/ 7 http://www.123india.com/ 7 http://www.elib.org/ 7 http://www.ezilon.com/ 6 http://www.epooch.com/ 6 http://www.christiansunite.com/ 6 http://www.toronto1.biz/ 6 http://www.canadianeh.com 6 http://www.aardvark.co.za 6 http://www.nzs.com/ 6 http://www.aigam.com 6
This thread pops up very regularly... The truth is, that directory submissions may improve your page rank, which in turn may improve your SERPs. Are they essential: Certainly not. Are they important: That depends on what you're trying to achieve. Generally, for one primarily concerned with getting to the top of Google's rankings, submitting a site to endless directories is probably a bad way to spend precious time -- but, if you have time to waste, it can't really hurt, either... Traffic from directories really depends on what you're selling, but is generally going to be a fraction of what you receive from Google/Yahoo/Bing...
In my view directories are not important, nobody search the site from directories, social networking is easy to get quality backlinks, also no reciprocal & free