In most (but not all) professional environments managers move up from the ranks of workers. Managers are familiar with with the jobs of the people they are managing. In IT, while a manager may have come up through the ranks, the technology and issues that their employees are dealing with change radically in a short period of time so the manger is no longer in touch. Furthermore, systems are now so complex, that its not possible for a manager of multiple applications and functions to be able to even partially understand their operations.
So based on this assertion, one of the major roles of the IT manager is to be a central point for the business to make requests, and the technologists who are going to do the work to evaluate those requests.