On my car, 10% equates to about 25 miles rated range. Why do I have such a large buffer?
1. It is easier on the battery, promotes longevity of the battery
2. It maintains the performance of the vehicle - highway speeds
3. It covers for elevation changes
4. It covers for weather variability (rain kills range - tires pushing water out of the way uses a lot of energy)
5. It covers for the extra weight in the vehicle (that always happens on long trips)
6. Tow trucks are expensive.
7. My wife freaking out in the right seat is even more expensive