Concept of leave

To start with, here is a dilbert strip trying to explain what usually the managers think about the output we produce.

I was just wondering why my office has a concept of Leaves and vacation.
Here is a background:
I work in a product company and out here, every single person has different sets of responsibility and areas of specialization. Net result is that a manager cannot offload one engineer’s task to another engineer easily. (unless he accounts for the time that would require to train that new engineer to get familiarize with the setup.)

So the net result is that an worker here is responsible to get the work done. It does not matter how many long hours he takes, how many weekends he works (and yes, there is no overtime). If I want to take a vacation, then I am expected to work a little harder before the vacation and after that so that the project deadline does not suffer because or me.

This phenomenon makes me wonder:
Since I am responsible for my output and that does not suffer because of the fact that I had taken a week off, should my company deduct no of leaves for that period?
How is it in your office?