The number of incoming links is one of the most important ranking factors for the major search engines (SE)as Google. By submitting your site in a lot of directories, you will significantly increase it.
It is a way to be quickly listed in SE because they visit regurlarly already listed sites. It is more effective than a direct submission to Google.
You multiply your chances to be founded by search engine robots.
and ... you are visible for the visitors of a large number of sites !