Articles are shorter than books. You may only want to familiarize yourself with a topic. Articles often address very specific aspects of a topic. For example, you might want to know about alcoholism among college freshman attending California universities. It's likely there aren't any books on this very specific topic.
More articles are published than books, therefore, there are more articles to choose from.
In addition, articles cover a broad array of topics in just about every area of interest. Research Instruction Online, University of Arizona Library.