My "aha" moment on this was realizing the difference between estimations and commitments. If there is some probability distribution of how long a task will take, an estimation gets the median of that distribution. On the other hand, a commitment gets at least the 95th percentile of that distribution. Estimations are useful for general scheduling and resource management, while commitments are useful for having others waiting to get started as soon as a task is finished.

The biggest problem I run into is people not being clear (or not knowing themselves) whether they are asking for an estimation or a commitment. "How long do you think X will take?" sure sounds like a request for an estimation, but if they follow it up with "Great, I'll tell the customer it will be done by then.", they clearly were looking for a commitment instead.