What is dominance?
Loosely speaking, dominance is the display of prowess -- the showing of physical markers or displays of strength or power, to avoid mortal combat. For males of all species, dominance is a non-lethal way of settling disputes over who has claim to females.
Picture two stags in the forest. One has big antlers, one has teeny, tiny antlers. By just looking at the stag with big antlers and going 'damn, that dude has bigger antlers than me, he's going to kill me so screw this, I'm going home,' the little stag saves his own hide.
But why did the bigger stag let the weaker one live?
In other words, if he's guaranteed the win, why not just kill the littler one and get it over with?
Fights to the death are costly. Always. And not just to the loser that gets his ass kicked. If the likely winner gets involved in physical contact, he's still placing himself at very real risk of injury or death, which may weaken him for the next dominance interaction or future conflicts with other males.
Goes back to the basic game theory, aka hawk versus dove. It's a known entity vs an unknown enemy. As in, even if you know you are a hawk, you don't always know if you are fighting a dove or a hawk next.
Plus, he could randomly injure himself. Comes back to chaos theory. The "unknown factors" in the universe (i.e the chance of the stag tripping his little hoof on a rock and goring himself to death) make any potential conflict costly.
So, he lets the other, weaker stag live.
The benefits of dominance for the winner? Obvious, really. He gets the females, at no deteriment to himself. What's in it for the loser? He lives to see another day and may get a chance to dominate at a later date.
So, submission is also important in nature. It just serves a different function to dominance. The purpose? To protect the weaker individual and give them a chance at future dominance.
The most interesting conclusion we can draw?
Submission is a choice.
So, why do males fight to the death?
Because they think are equals and all non lethal means of settling the conflict have been exhausted. Or, if the loser feels there is nothing left to lose and is willing to take a punt, ignoring the superior dominance display of a stronger male.
Basically, dominance only becomes physical lethal conflict when the two parties disagree on who wins. In this scenario, submission is not an option - the lesser male has death forced upon him...