As this article from CIO explains, it’s all about expanding your skill set:
"Most professionals think they have to change jobs every three years to get ahead. But you really don’t have to move to a new job or company to advance your career. Chances are, your current job offers challenges and opportunities you haven’t yet tapped. By taking on new assignments in your current position, you can expand your skill set and develop your leadership capabilities—and thus your marketability—without spending all that time and effort job-hunting.
"The key to making progress in the workplace and in one’s career is to identify and take on developmental assignments. These are roles and activities that provide opportunities to learn new skills, expand your knowledge base, try new behaviors and improve on weaknesses. Because they usually involve an element of challenge or risk, they stretch you out of your comfort zone. A developmental assignment might lead you to work that is broader in scope than what you are used to, such as a project involving more people or coordinating with groups across the organization.
[snip]
"Consider temporary assignments outside your job description or department. What new projects need an additional team member? Is there a task force that could benefit from your functional expertise and, at the same time, give you a broader perspective on the business? Are there annual events you’d like to plan that might help you improve your project management skills? Would a temporary assignment in another department broaden your knowledge of the business?"
Some pretty good guidance in the whole article; be sure to check reader comments too!