When you graduate and join a large firm, firms generally hire you for a specific practice area (e.g., litigation or corporate). There is a business need that justifies placing you in a particular group, and the firm is expecting that you will be able to help service specific clients in that particular group.
Practice groups within a firm often operate like separate and independent firms, and many have little or no cross-pollination among practice groups. Many junior associates have the misconception that, because a firm has numerous practice groups, they have the ability to freely switch between practice groups because they all work for the same firm. It is important to keep in mind that the firm is a business and a firm views its associates as investments. For each junior associate, a great amount of money is invested by the practice group - through both recruiting and training - and it takes a good deal of time before the firm reaps financial gains from this investment. Certainly the best time to experiment with practice areas is as a summer associate when the firm has invested less money in training you. There is more at stake (because the firms has continued its investment in your career) when you are an associate.