You see, back then there was a much smaller number of web directories out there. Because the search engines were much different then (link popularity was not a major part of their algorithm yet), there was not a need for the low level directories which seem to exist solely for the purpose of increasing one's pagerank and link popularity. The major engine of the time, Excite, used such factors as keyword density and meta tags almost entirely when ranking a website.
It was not untill webcrawler came around that link popularity became important. What few directories there were usually sent a substantial amount of traffic to your site, comparitively speaking. If you were listed on one of the major directories, depending on your niche, you might recieve enouph traffic from one of them to practially make a living (in those days the Yahoo directory was set up differently so that if you were in their directory you recieved droves of visitors from both the category and you recieved high priority on related searches).
Nowadays link popularity and pagerank are major factors in search engine positioning so there is amajor need and desire for more links, thus the greater number of web directories. They vary greatly in quality: some are human edited with strict standards of approval. Others approve every listing, even from low quality sites that have little content. Will these directories help you from an SERP standpoint? Well, it depends, in my opinion, on how targeted the category is.
You must spend some extra time finding a category with similiar content as your site or you may gain pagerank at the price of relevence. And some reports suggest that the major search engines may be emphasizing relavence more and more in the future.
And of course there are still some high quality directories out there. Most people are familiar with DMOZ.org and Yahoo.
These are probably still the most important. But there is also JoeAnt, Goguides, Buzzle, Gimpsy and Splut, among several others. And more are rising in relavence. To increase the relevence of your own site in the eyes of the search index sites, augment your directory submission with forum postings in related forums and write some articles on the subject matter of your website, then submit them to the article directories with your link in the resource box.
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