There are different reasons. For me I have to make this choice:
1) Require 20 hours of my free time to complete a job.
2) Require 10 hours of my free time to complete a job.
The choice is clear. #2 is far, far better. Thus I choose #2: procrastination.
Now the real question is: why is procrastination far less work? Different people have different answers and often it is a combination of them. For me the major factors include easy distraction, lack of planing if you don't procrastinate, and perfectionism. I think two of those should be clear, but I'll go through them all. I'll use an example of writing a 5 page paper.
[*]Easy distraction. When I have no pending deadline, I have more difficulty concentrating. As I am researching, writing, and editing there is the constant thought of things that would be far more fun to do. That means I can only put 50% of my mindpower into the paper. Obviously the result is that I work less efficiently and it takes longer. I often give in to those thoughts and turn on the TV while working. When that happens I'm half watching TV and half doing work. Again, the work takes twice as long. With a looming deadline, none of these thoughts enter my head and my work gets 100% of my attention.
[*]Lack of planning. When I just dive right into a paper, I really have no clear-thought plan. Did I really find all of the best references? Did I really think the logic through? Did I really have the best order for your paper? The answer is often NO. Instead, if I procrastinate, I can think about this stuff as I sit in traffic or as I shower. This time was wasted in the past, but now is productive. Procrastinating the writing gives me sufficient time to find the best references, to go through my logic finding holes, and to see if the paper layout makes sense.
[*]Perfectionism. I have a problem where I must do everything perfect. The result? If I finish the paper a week early, I will still spend every evening reading, rereading, and reworking the paper to death. This often adds many, many useless hours of work on to the project. With a looming deadline, this perfectionism has no chance to rear its ugly head.
That said, I've never been late on anything that I procrastinated and I've never turned in sub-par results due to it. I only procrastinate until I MUST do the work. My perfectionism won't let me procrastinate further. End result: same quality work with far, far less effort.