At this point, it's so common that most people don't even question it. Of course the game allows for a character to be killed in combat; that's how combat works. It's realistic! It adds a sense of risk that heightens achievement! While this may be true, it's worth noting that most other storytelling mediums do not have regular character death.
"What about video games?" you ask. Or, "What about Boromir?" "There are countless examples of characters dying," you point out and, since this is the internet, you probably add "and you're an idiot for saying otherwise."
With the exception of video games, which are a unique circumstance that I'll tackle in more detail later, it is hard to name a story where the primary protagonist dies before the end. It's just as hard to name a story where a protagonist dies in a meaningless, random way. Death, in most stories, is used not as a failure but as a way to heighten drama and add to the story.
Yet in tabletop roleplaying games it's comparatively common and often random. Why is that?
The Reasons for Death
When you break it down, most games include death for two reasons: realism, and sense of risk.
Death for Realism
If you get into a sword fight with a rival or enemy, there is a very good chance one or both of you will be seriously injured or killed. If someone is shooting at you, and you decide to stick around and shoot back rather than fleeing, it's entirely possible you'll catch a bullet and perish. Death as a consequence for combat is just realistic.
Outside of combat, death could still be a very real threat. Traveling through a hostile environment without proper provisions could lead to death to exposure or starvation. Attempting to climb a cliff, tree, or building without a safety net or, worse yet, without proper training could mean falling to your death. Again, this is a logical consequence given what we know of the real world.
Often a player character is made to be more resilient than the average person, is given a large buffer of hit points or plot immunity. Yet, when that buffer inevitably runs out, death is still on the line; you can only push your luck so far. The harsh reality is that everyone dies, eventually.
Death for Risk
Accomplishments without risk of failure are meaningless. If students had unlimited chances to take exams, final scores become useless measurements as everyone will, eventually, achieve perfection through trial and error.
Additionally, greater risk carries greater reward. This is why gambling is so insidious: putting more money on the line means greater potential earnings, but it also means a greater rush when you beat the odds. The disparity between the worst that can happen and the best that can happen directly impacts the excitement achieved in success.
The logical conclusion is that when your player characters risk death, which in turn means the players risk losing all of their investment in those characters, victories are that much sweeter. When another character dies, the fact that yours did not makes you all the happier.
Death in Video Games
These two reasons for death are both used in another medium as well: video games. Particularly "death as risk" is a common part of most video games, from Mario to Call of Duty. By examining how video games handle death, therefore, we can learn to apply those same lessons to the tabletop.
Death is often used as a failure state in video games. If you miss the jump you fall into a bottomless pit and die. If you fail to kill the enemies in time they will eventually kill you. If you don't escape from the collapsing building quickly enough, you're caught in the explosion (or implosion) and die. At a glance, this seems to be identical to how death is handled in tabletop games.
On closer examination, however, death in video games is different in one key way. Death in video games does not mean the end of that character.
When a player character dies in a video game, the player is presented with a game over screen. Then, he or she is taken back to a previous point, prior to the death, and allowed to keep playing. This earlier point could be the last checkpoint, a previous save file, or even the start of the game; however, in the game's story the death is erased, the character is alive again, and the events continue to unfold from that point on.
No matter how many times you get your player character killed in a video game, when you eventually reach the ending the canon is that the character did not, in fact, die at all. Death is a temporary state, quickly erased in favor of allowing the player to continue the game.
There are exceptions, of course. In games with large casts of playable characters, sometimes the game will allow one or more of those characters to be killed off for good without forcing the player to restart from an earlier point. In other games, a save file (or even the entire game) may be erased if the character dies, resulting in a death that is more or less permanent.
But these exceptions are just that: exceptions. Most video games will waive realism in favor of player entertainment.
Yet in tabletop roleplaying games, death is almost always permanent. Even in settings where resurrection is possible the character still died; he just "got better". More often than not, the death of a player characters means the player is done telling that character's story and must begin anew with a fresh character. If all player characters die, often that means the end of the campaign itself.
Of course, it is far more jarring to introduce "save game" features to a roleplaying game, or to allow characters to respawn. The takeaway from video games, then, is to instead substitute the consequence of death for something less permanent: capture, injury, humiliation or the like.
Why is Death the Default?
In some games, it makes sense for death to be on the table. In a horror game, characters can and should die, in order to add to the sense of despair and inevitability. In a survival game, never having a chance of dying seriously undermines the theme and tone.
Yet death is not always appropriate. In a heroic action-adventure game, the protagonists are supposed to be larger than life; how can you be larger than life without also being larger than death? In a game about characters' stories, death means a sudden and unsatisfying severance of one or more major plot threads.
So why is death almost always assumed to be a possibility?
My argument is that it shouldn't be. Or, more accurately, in certain games it should only be possible at dramatically appropriate times.
In the Lord of the Rings, Boromir doesn't die to a random encounter with a bunch of goblins. No, he dies in a heroic sacrifice to buy his friends time to escape. In The Princess Bride, when Westley and Inigo duel, Westley doesn't end the fight by taking Inigo's life; Iniqo still has more story to tell. In Star Wars, when Luke and Darth Vader finally meet, Vader could have easily ended Luke's life. Instead, Luke only loses a hand and the plot is deepened.
So the next time you're running a game about heroes doing heroic things, and that random kobold rolls well enough to kill one of the player characters, ask yourself: would this character's death at this moment be appropriate to the type of game we're playing and the type of story we're telling? If the answer is no, consider an alternative.