1. You get fitter
Although it may seem counterintuitive, gains in fitness actually happen when you rest, not while you're training.
2. You reduce your injury risk
Training causes micro-damage to your body's tissues and without adequate rest and recovery between training sessions the body begins to break down.
3. You can recharge mentally
Incorporating a regular rest day or a very easy day into your training will enable you to mentally recharge your batteries as well as physically.
4. You can train more consistently
Proper rest and recovery will reduce your risk of injury which will enable you to train more consistently
5. You can train harder
By giving your body adequate rest and recovery you will be able to adapt to and absorb more training over time without running the risk of overtraining.
6. You will race better
There is virtually no fitness to be gained in the final two weeks before a race as it takes at least 10 days to two weeks to reap the benefits of any given workout.
7. You can periodise your training
In order to produce a peak performance, you should periodise your training into different training blocks including rest and recovery.