Why do you think it is important to take an average resting heart rate versus relying on one value alone?

An average resting heart rate allows the doctor to approximate how healthy the heart rate and blood pressure are in comparison to the general population. Relying on one value would mean that a doctor would be comparing people's health to the health of one person. If that person is healthy, great. If they have a high or low heart rate, it could mess up the entire idea of health in relation to the heart, and cause many issues.