The human body is meant to move. It is not so far fetched that if the body is meant to move, that not doing so will cause malfunctions. We find this in nature time and again. You even see in in machinery. When a car sits for a prolonged period of time it develops problems when you finally try to start it. Here's a brief overview of how human movement impacts the body... some of which may surprise you.
Human movement improves hand eye coordination, balance, bone density, and blood circulation, blood composition, and energetics. A lack of human movement has the opposite effect and results in various diseases such as diabetes, osteoporosis, sarcopenia and heart disease just to name a few. It slows down the ability for the body to recover and regenerate on a cellular level. This then increases secondary aging.
Human movement and exercise help youth in physical development, as well as develop a healthy self-image and confidence. For people of all ages it helps alleviate depression and promotes emotional well-being. It promotes hormonal balance, which additionally impacts our moods, and our body's ability to heal.
Exercise and human movement improve our immunity. There's a fluid called lymph that helps eradicate viral infections found in and around the muscles. Human movement then helps circulate the lymph which allows it, to do its job. The improvements in physical health that increase our breathing and heart rate also make us less susceptible to viral respiratory infections.
Exercise as we age helps reduce falls, fractures if we fall, and frailty. Heavier lifting... such as taking out the trash or moving boxes at home can have dire consequences if we do not keep our muscles strong. When we become frail the heart rate dramatically increases when we lift something that is "heavy" compared to our physical ability. Such instances can result in heart attack, back injuries and falls. Furthermore by keeping dense bones through exercise if you happen to fall the likelihood of breaking a bone is dramatically reduced. Staying healthy in this way ensures you can live unassisted for a longer period of time.
Exercise and human movement are associated with improved cognitive function. This is true of those over 65 years of age, and is also true of youth. Studies show children that regularly play tend to out perform their sedentary counter parts academically. There is some research looking at exercise and its impact on ADHD.