Hand to hand combat is a physical challange that most people never learn to fully appreciate.
The subtle art of thrust and parry combined with the strength, endurance, tactics, and fortitude to see it through, or, as one of my instructors once said "chess with muscles".
It's more than just the act of hurting another person, or even defeating them, as the true aim of
combat may even be achieved in defeat, and that is, to
satisfy the "lunch hunting" instinct. Our species is predatorial. Modern society eliminates the need for the chase and kill and these
urges need to be fullfilled somehow.
Hand to hand combat meets this need, be it any of the martial arts. The physical effort in learning the skills, and the fitness nessesary to use them adequately, equate to learning to hunt. The actual chase, the chance that your lunch may turn on you, evokes the same fight or flight response a worthy opponent does. This all helps the combatant appreciate the fragility of their own life and satisfy their hunting intinct.
Total awareness of your self, and having your body function as if on automatic pilot, is an experience that cannot be duplicated in any other situation with the same degree of intensity. The knowledge that your opponent will hurt you (usualy badly) if your concentration lapses, and the skill to maintain total relaxation (like your not concentrating), is so incredibly uplifting it's nearly religious. I've heard this described as meditation in motion, and the Japanese even have a word for it: satori. This is the ultimate state for any athelete in any endevour. It is what all combatants strive for whether they realise it or not!.
It's also addictive (so is the pain, but that's another story), but not in a detrimantal way. It gives you clarity of mind (through freedom of distractions) in your everyday life but with the anticipation that soon you will be in combat again.
The other main benifit is the self confidence achieved through knowing you can take care of yourself.