My Shoulders started to get bigger after learning Muay Thai, maybe its also the Military Presses that I do. Kickboxing benefits you in every aspect as a human being, just like any form of Martial arts.
In terms of fitness, you'll build tone, get some muscles, Kickboxing is a anaerobic sport, just so you know. You'll soon learn to become faster with your punches and kicks. I started Kickboxing (MT) with a flabby body, that, along with weights and cardio had me lose my gut, I built muscles, and I was punching faster and more powerful. My endurance levels are better than before.
In terms of mental aspects, it'll make you more confident, just like all forms of exercise. But me personally, I like to look at Kickboxing as something that'll eliminate fear. I don't spar, but if you do spar its even better. Just facing people you don't really know that well in your training school, you don't know how hard they'll kick or how fast they'll punch... we have this thing that if we get hit by the other guy during our pad work, its our fault, and it should be for no being ready. I went in the gym all nervous and scared of people, but now, I don't care who I train with in the gym, I'm not even scared of getting hit.
Hope that helps. Do kickboxing, its a cardio alternative.