Rest days are vital in a workout regimen as they allow the body to recover and repair muscle tissues. The muscle-building process occurs during rest, where the body adapts to the stresses of training. Not taking adequate rest can lead to overtraining, increasing the risk of injuries such as strains or stress fractures. Rest days can also improve overall performance, as they ensure optimal energy levels and mental focus during workouts. Additionally, these breaks help maintain motivation and prevent burnout in training programs, contributing to long-term fitness success.