There are two major reasons why people use article directories for traffic generation reasons.
Firstly it is to create back links pointing back to your site. Search engines love useful content and they love genuine links and so when search engine robots come across useful articles with links pointing back to your site, chances are very high that you will end up being rewarded by traffic coming to you directly from search engines for your most prominent keyword phrases. What you need to do here is to ensure that the article directories you use for this purpose have a high Google Page Rank (a PR of at least 4 or above is excellent).
Read second and very important reason.