Work in progress
Have a look at the following image:
As you can see, as you increase the utilization rate of people, the amount of time it takes to complete something on average increases, and after about 80% increases dramatically. If you are trying to estimate work and you have a high utilization rate, then which number would you use - the one represented at the lower end of the curve, or the one at the high. The difference is “orders of magnitude” on the estimate.