Because when you measure your level of flexibility you will know how good you actually are in terms of an increased range of motion and this will eventually help you if you want to play in professional sports - professional athletes namely value flexibility a lot as a characteristic (with it of course being more important at some sports, compared to others).
You should measure you level of flexibility in order to know you limits in terms of stretching your body and immersing yourself to different sports that require more physical effort. Flexibility here refers to the absolute range of movement your joints and muscles will come across. You will be in motion.
I think the person got some "fats" in there serving and havent burn it through exercising so its stored in the body as body fat which will make the person gain weight.
