I'm actually very vigilant about not procrastinating, because I know what it can do in a life. (It's like a substance abuse problem - once someone starts doing something a little bit, it can get way out of control.) Having said that, however, I'm had my moments of procrastinating (and lots of them)...
First reason: Too tired sometimes. Sometimes it's just better to do something later, when a person isn't too tired to do it right.
Second reason: Saturation point with doing stuff I hate. If there's something I hate doing, and I'm feeling as if I've done so much other stuff that I hate doing; I'll get into a "saturation point kind of thing" and think, "I'm just not doing this one more objectionable thing for now. I'll wait until I'm "mentally nurtured" enough that I'm replenished and more able to deal with doing something I hate.
Third reason: There is actually, and occasionally, a little wisdom in the motto, "Never do today what you can put off until tomorrow." Sometimes what was "SO important" yesterday turns out not to matter much today. In fact, if a person procrastinates long enough, some problems either get resolve on their own or else go away! LOL
Fourth reason: One man's "procrastination" is often just another man's "re-scheduling". :) Being efficient and organized, and always being on top of things and doing the right thing, are great; but life has a way of showing us that sometimes one of the most important attributes someone can have is the ability to be a little flexible.
A Ralph Waldo Emerson quote is "A foolish consistency is the hobgoblin of little minds, adored by little statesmen and philosophers and divines." I tend to apply that quote to those times when I decide to be a little flexible, or times I choose to reason out whether, exactly, procrastination might be the wiser thing - and then I make myself feel better about procrastinating by telling myself I must have a mind "big" enough not to be crippled by those "hobgoblins of a foolish consistency". LOL
(I have a friend who has lost lots of money as a result of not wanting to put until tomorrow what he thought would be better if he did immediately. Sometimes the urge to procrastinate may actually be "Nature's way" of letting us know we ought to hold off on some thing. :)