The reason that task management is so important in relation to setting proper priorities is when things change, things come up, or emergencies arise (it is just a matter of when), you can quickly identify a new or changed priority and put it into your mix of tasks.
A very useful method that I have found is the A, B, C, D, E method in order to identify the importance of tasks. An A task is a top priority task that needs to be done today or in the next 24 hours. A B task is a task that needs to be done between two days and the next week. A C task has the priority between one week and one month. D, however...