Deadlines are important because they add an external reality pressure to your internal motivation.
Did you ever hear the advice "live as though death was like a parrot sitting on your shoulder"? Death is the deadline for our life goals. If we had no deadline we would be able to put everything off forever. Death provides the external reality pressure.
We can not make any sense of our schedule or our activities unless we are forced to put them into real time. Otherwise we would be able to put everything off until tomorrow. Manana!
Deadlines, by providing an external; reality pressure, force us to fit our activites into a definite time frame and accomplish our objectives. They also make it possible for us to coordinate our activities with others in our lives and organizations.
If your work setting does not provide deadlines, you have to learn to set your own.