1. Rest is when the body rebuilds itself.
Everything is done best in moderation. Working out too much can be counter productive. We need to give our body adequate time to recover. Recovery is when muscle growth occurs. Recovery is impossible without rest.
2. Adequate rest leads to sustainable workouts.
By including one or two days of rest per week, individuals are much more likely to stick to their workout regiments. The burn out rate drops and people stick to their goals because they are not overworked.