Predicting how long a task will take
If someone is familiar with repeating a task, it becomes a habit. Routines within a habit take less brain power. With less conscious effort needed to do it, someone would most likely underestimate or gloss over what the task consists of. 

This is just one possible interpretation; there might be other reasons as well.

The evolutionary advantage would be that we need less mental resources to deal with things we are familiar with. We would say, struggle to learn to peel a potato at first, but after a few dozen/hundred times, we do it effortlessly. The brain will try to optimize these things into a habit loop to conserve resources needed to do things. Without habits, we would spend too much time on 'simple' things like recognizing a person's facial features.

Source: Duhigg, Charles. The Power of Habit: Why We Do What We Do in Life and Business. Random House, 2012.