Temporary employments to gain work experience. Generally internships are done during college.
These shorter term employments serve two primary purposes. First, it helps the employee gain on-the-job training in their chosen field, which is useful for finding fulltime positions. Second, it allows companies to have an extended interview process to help ensure they hire the right people into their fulltime positions - an internship gives them a chance to evaluate potential fulltime hires for considerably less cost.