Using a delegative style with a worker who knows more about the job than you. The employee needs to take ownership of her job. Also, the situation might call for you to be at other places doing other things. Type of task. Laws or established procedures such as OSHA or training plans.
Positive leaders uses rewards, such as education, independence, etc. to motivate employees.
Although it has its place in a leader's repertoire of tools, it should be used carefully due to its high cost on the human spirit. Negative leaders act domineering and superior with people. They believe the only way to get things done is through penalties, such loss of jog, days off without pay, reprimand in front of others, etc. They believe their authority is increased by freighting everyone into higher lever of productivity. Also note that a leader is not strictly one or another, but is somewhere on a continuum ranging from extremely positive to extremely negative.