Directory submissions can eventually lead to an increase in traffic by way of improved rankings that come about from the increase in quality inbound links that you get to your site.
When search engines find your links in the directories, they tend to classify your site based on key terms or phrases used in the link/anchor text pointing to your site. Since you can control your link/anchor text by way of the titles you enter, you can use it to target specific keywords or phrases. This will increase the chances of your site appearing high for searches targeting those words or phrases. The higher you rank, the more popularity you gain and the more incoming traffic you will attract.