Including rest days in a training plan is essential for several reasons. Firstly, they allow muscles to recover and repair from the stress of workouts, reducing the risk of injuries. Secondly, rest days help maintain motivation by preventing burnout from intense training. Additionally, they support muscle growth and overall fitness progression because rest is crucial for hormone production and restoring energy stores. Lastly, rest days can improve overall performance and readiness for future workouts, leading to better results in the long term.