I will speak in cliches, sorry about that.
Your body is your temple.
Healthy body, healthy mind.
Etc.
Basically, it's important. Of course, you might decide to not be exactly fit, but keeping it healthy is imperative.
Importance of physical fitness?
When you're physically fit you will be more contented with your outer appearance.. Tendency is you will strive hard to other things that matters to you like studies, improving your skills/talents , etc. In short, you will enjoy your life more.
Physical Fitness is very important for our good health,if people who are both, physically as well as mentally fit are safe from medical problems .Fitness, along with proper nutrition, is important for maintaining a healthy lifestyle.
Physical fitness affects in all things, like in mental fitness too. So its better to have proper physical fitness with all proper diet and workout.