Why slack off at work? Because you want to have value in your future.
Let’s start with your paycheck. Paychecks are very predictable. For example, the probability of my next paycheck not happening is much less than 1 percent. I can therefore claim all the value of that paycheck now, and regularly do so in the form of loans and credit. I have already claimed the value in my future paychecks, so there is no value in my future from this source. (What value there was to be had in the future has effectively been moved to the present)
This is problematic; I rely on gainable value in the future to guide my actions today. Future value I can count on provides no guidance, no purpose, to my actions. I need to increase the value available to be gained in the future.
One way to increase the value available in my future is to do more. Get a second job, join social groups, have a hobby, make some thing. However, most of these have the same problem as my (primary) job: they are too certain. As soon as I am established in any of these activities, I can count on that future value in the present, and there is very little value available to me in the future.
Another way to increase the value available to me in the future is to reduce the certainty of that value. Slack off at work (maybe I will get fired!), be non-committal to the social group or hobby (maybe I won’t go today!), only occasionally work on the thing (it might get done someday!). The uncertainty reduces the present value, but, crucially, makes that value available to be claimed in the future.
This same idea can be applied to procrastination. This is left as an exercise for the reader ?