Every aspect of your health is connected. This means that in order for you to be truly healthy, you need to maintain healthy habits in each area: Nutrition, Fitness and Sleep. If your fitness level is low, it will negatively affect your nutrition and sleep.
A lack of exercise will obviously make you weak, as you aren’t developing the muscles you need to go about your day optimally. In fact, without exercise the muscles you currently have will deteriorate.
More than that, though, a lack of exercise will make your body weak when it comes to preventing disease. You’ll be less prepared to fend off diseases, be more likely to have heart disease and many other serious problems.
Finally, since exercise raises your metabolism, a lack of exercise will make it easier to gain weight.