Semi-seriously, e is a number defined by recursive growth.

So if you estimate to Level 1, and then break down to Level 2 sub-tasks, then Level 3, your estimate will be out by the same amount each time. and your total error will be a multiplicative series.

If you have an infinite number of levels - which is reasonable for most real-world projects, although this may not be obvious in the planning stages - e will get you nicely in the ballpark.