Power from a punch/kick comes from the core. Kickboxing strengthens your core and define your muscles with every punch and kick.
Being an active sport, kickboxing will quickly increase your heart rate and strengthen your heart muscles. It will provide increase stamina and help control your blood pressure.
kickboxing incorporates balance and stability with quick retracting movements. Having proper balance and stability will help your body generate more power.
Agility is important, sometimes more so than muscle strenght. With every kick/punch you tone and strengthen your body to respond faster in a way pure weight lifting cannot accomplish.
As we become more sedentary, we lose flexibility. The continual flow and twisting moves from kickboxing will help restore flexibility.