There are so many criterions that search engines follow to determine a sites rank for certain key or search words. One of these includes links to the site. The page rank of a site is directly related to the number of links it has. The more links you have pointed to your site, with some variation of the key phrase you want to rank high for, the better ranking in the search engines you will receive.
Relationship between directory submissions and search engine rankings
One way links and search engine rankings have a great cause and effect relationship. Think of it as a business referral system. The business with the more referrals is most popular and hence, search engines rank them higher. The reason behind it, as many put it, is that the more links you have, higher are the chances of a visitor coming to your site through these links!
Again, the search engines send out spiders to the links they find in the directory pages. This will bring the search engine spiders to your site and will get your site pages added to the search engines indexes automatically.
In future, the new search engines will also follow the same procedure. Hence, you wont have to...