Most of the time, department heads will have a hard time trying to take on all tasks that are given to them to meet the expectations set by an organization. This pressures them into making up for time, pointing towards extended working hours, stress, and making immediate managerial decisions without cause. These result into an issue where their positions may eventually be in the hot seat, and in such cases, become a measure for incompetence and inability to handle their jobs at certain levels.
For the mature managers, they assign and delegate some minor duties that they deem fit for most people who can decide, lead and manage certain tasks. In these cases, duties that can be foregone and left to the jurisdiction of some department employees will certainly lighten the load and allow the superiors to maintain focus on the necessary areas crucial for the total performance evaluation. Aside from building the confidence of empowered employees, the total function of the department will prosper in such instances, since these selected personnel are given the opportunity to excel and shine for undertakings that may lead to further career improvement and development.
Here are some good articles to read on Delegating Work and Duties: