Articles are used for 2 purposes:
1. Bum marketing: You write a lot of articles that rank well in search engines for your keyword, and you get good traffic.
2. Getting backlinks: Yuo add your articles to different directories with your backlink and as a result your article gets you a number of backlinks on your website, helping your SEO.

Hope that helps.

You ask an excellent question. Article submission can be a powerful tool I've written and posted over 500 unique articles to literally hundreds of article sites and the best article directory site by far is Ezinearticles.com.

Essential what happens is that when you post your article it can get picked up by website and blog owners who need content for their site. So your article could get posted on multiple sites with your website link contained within the article. So if you write an article that gets posted on 50 sites, that's 50 new backlinks pointing back to your site.

Another warrior forums member has posted an excellent thread that will help you build links to your site and has a list of article directories and you can view their thread Here.

I've taken this one step further and started an Article Directory List & Comparison Table. Thsi doesn't just list the sites but also provides a useful rating system to help you focus your efforts. You can view it Here