There are a number of reasons why we need to think of a topic in terms of resource type:
Resource type is important depending on what type of information you need. For example, if you need a basic understanding of health disparities, a book or encyclopedia might help. If you need current information on evaluation of a specific program designed to reduce disparities, you'd probbaly use articles.
The resource type you need will determine how you search for relevant resources. For example, you can't identify articles using the library catalog (though the catalog will tell you what journal titles we have). The options within the By Type of Resource menu will help you pick the best search tool to find the right sort of resources.
Sometimes your instructor will tell you what sort of resources. Check your assignment carefully. Do you need to use specific types of resources or can you use any type of resource?