People post articles for free in hopes that doing so will generate a lot more traffic on their own site.
Most article sites allow the writer to post along with the article at least a URL to their own site which the writer hopes will be followed by readers. In some cases, the writer "donating" the article can post a short bio as well. Most of these forums try to create a social network as well. The more active you are on their forum, the better the chance someone will visit your site.
A Google SERP yields about 108,000,000 links for article site so you have a lot of research ahead of you. Most people who do this immediately see a jump in their visitor stats. Check out some of these for yourself by visiting Avangate, a website that posts a link list of 50 sites that welcome articles from contributors.
Thursday3Some: Tips To Choose Wisely
At most article sites, you must create an account. Here are some tips to help you choose the right site for your contribution.
2. Check out the quality of the articles on the site. Do you really want your work in the same arena as content that is full of grammatical and/or factual errors?
3. Check how easy it is to use the site's software in order to upload your articles. Some sites have software that is cumbersome while other sites are much more user-friendly.
A writer in search of an audience must kiss a lot of frogs before finding a prince of a promotion idea that works.