A safety margin is room reserved for the fact that people and conditions are not exact.

A margin exists because people and conditions are never exact. It is not wasted capacity or excessive caution; it is the space reserved for the variation that is certain to occur, and the entire function is to be unused most of the time. Anything running with no margin is running on the assumption that nothing will vary.

Apply this to your own schedule, which is the most common place people delete their margins. A day booked to the minute has no room for the ordinary variance of life, and will therefore fail regularly. Leaving gaps looks like inefficiency and is actually what makes the plan survive contact with a normal week.