If you want to lower your risk of heart disease, cancer and diabetes (as well as many others,) living a healthy lifestyle is key (also, don't smoke.)
Have more energy.
If you want to make it through the day without collapsing at the end, living a healthy lifestyle can help you have more energy and get more done.
Have healthier skin, nails and hair.
Not only does exercise make your body look better, it also makes you prettier.
Live a longer, more fulfilling life.
Even if becoming a centenarian isn't your goal, living a fulfilling life should be. Nobody wants to be on oxygen, have to use a walker, or be in and out of the hospital at the end of the their life, and you can avoid those by living a healthy lifestyle.
Your emotional and physical health go hand in hand. By taking care of your physical health, you'll have a better handle on controlling stress and your emotions.
Would you rather pay the money to get an expensive surgery because you didn't take care of yourself or live a healthy lifestyle?
Be in less pain.
When your joints, digestive system and immune system all work better, you have a lot less potential for pain and illness.
You can't run the Boston marathon, hike up Machu Picu or climb Mount Everest (dream big!) unless you're in shape to do so.
While living a healthy lifestyle won't completely cure insomnia, you can partially get rid of some of the factors that prevent people from sleeping: sleep apnea and GERD, to name a few.
While working out and eating healthy is initially an investment of time, in the long run, it pays off. You spend a lot less time filling prescriptions and at the doctor, and more time living.