The first benefit of good nutrition is giving your body to do the job it was designed to do- which is to be healthy! The easier it is for your body to function normally, the more it can do all of its work without having to 'put out fires'. This means it is easier for you to think, walk, talk, sleep and do all of the hundreds of things you do daily without even noticing.
How do you know what good nutrition is? Think of foods that are not processed. This means the fewer steps food goes through before it's on your plate the more nutritional value it carries with it. The fresh peach, or plum or carrot is much easier for your body to break down and digest. Compare to a prepared entree that probably started out fresh, went to a processing factory and has now gone through many chemical processes to get to you. It is older, less fresh, and not quite as alive as that fruit or vegetable is.
The second benefit of good nutrition is that it's easier to recover from illness or injury. When you are ill, your body goes into overdrive protecting you, and producing many different hormones and chemicals. This puts more 'stress' on the system. When the under mechanism (your body) is reliably sound, the stress does not last as long, and does not put as much pressure on your body as if you were ill. Think about the healthiest person you know. Someone who eats brightly colored fresh fruit and vegetables, exercises, etc. Other than a serious illness or injury, notice that they are able to bounce back more quickly from minor, or everyday illnesses.
This does not mean that you have to become someone who is totally into raw food, or running marathons to be healthy. The bottom line is that humans have not evolved from hunters and gatherers to people that get the majority of their food from a box. The many wonderful (and not so wonderful) advances we have made in the areas of technology and food are not always to our advantage. Read the "End of Food" by Paul Roberts if you would like more information in this area.
The third benefit of good nutrition is reducing mind stress. If you are receiving an adequate intake of vitamins and minerals, both through food and supplements, your brain will work better. If your brain is fed, it's easier to make decisions, ranging from everyday to complex. An excellent book on this subject is by Dr. Mark Hyman "Fix your broken brain by healing your body first".
Good nutrition is essential to your health, and the good news is it's in your grocery basket- think easy, simple and fresh!