The answer is probably exposure. You need to know your target audience in order to do effective advertising. Directories usually help to find where groups of people with similar related interests gather. For example, if you submit a forum about music to a music category in a directory, you can find other music forums. You can go there and try to advertise your own music forum too inside people who have high interest in music.
If the link directory you are starting has a high quality content and many categories, it will succeed in the market. Once your website is a success then you get paid for it. How it helps other webmasters is they can build links and increase there rankings