Workflows allow you to set specific steps in publishing content. These steps can be tied to user roles and permissions.
For example, if you were to set up a workflow for articles you could allow some people to begin writing an article, and give them a button to send an article for review (but not to publish). Once they send it for review other people could have rights to review and edit that article, publish it, or send it back for rewrite.
You can add as many steps and user permissions as you need for your publishing process.