[I]t often seems to me that of all the good things in the world, the only ones humanity can claim for itself are stories and music; the rest, mercy, beauty, sleep, clean water and hot food...are all the work of the Increate. Thus, stories are small things indeed in the scheme of the universe, but it is hard not to love best what is our own...
-Severian, from Gene Wolfe's The Citadel of the Autarch
We are the only things in the universe, as far as we know, which make up and tell each other stories. We are homo narrans.
Does this serve some instrumental purpose that is opaque to us but may one day be figured out? Is it a mere epiphenomenon of our brain development and evolutionary background? Is it a gift with which we have been endowed by God, so that we might do him proper homage as the original creator - and hence storyteller - who began telling the tale of the universe at the dawn of time? Or all (or none) of the above?
In a sense it doesn't matter: suffice to say, when you're telling somebody a story, you are engaging in possibly the most uniquely human activity there is - even more so than music or the visual arts, which the behaviour of other animal species can sometimes approximate. Reading and writing fiction is not frivolous, time-wasting frippery. (Unless Louise Mensch is involved.) It's what makes us us.
I prefer to think of stories as the way we make sense of what it is to be human. For whatever reason, whether through a blessing or a curse, we are engaged in that project from the moment we are born until the moment we die, though none of us chooses it. If we attend to the matter of being human carefully enough, we may get some of the way towards a conclusion - though, as is the case with any conclusion really worth making, we could never state it explicitly. Stories, then, are a means of attending - a method by which we can hold up some aspect of human life to the light, turn it this way and that between forefinger and thumb, and see what we can see within it. We rarely get this opportunity in our real lives, because we are caught up in the business of living them. It is only in moments of quiet repose, when writing, reading (or hearing) a story, that we get the space in which to do it. And the fact that it happens obliquely, through what is in essence a metaphor that whispers its meaning to us without our conscious minds often even hearing, is only what gives it its power.
(I can think of no better illustration of why "the human brain is like a computer" is an utterly foolish way to think about ourselves than reflecting that, of all the works on politics, political theory and political philosophy I have read, it is Turgenev's Fathers and Sons that stands out head and shoulders above the rest in the understanding I feel it has given me about political belief. A computer would understand politics by reading a textbook on politics. We understand it best through stories - and the madness of our current politics is surely in large part due to the decline of reading of serious novels and biographies.)
If stories are the most uniquely human activity there is, then playing RPGs takes on additional importance. Unlike any other form of storytelling, a D&D campaign is both shared, and unpredictable. One doesn't just attend to what Dickens, Chaucer, Vance or Helprin has gleaned about this business of being human. One attends to what one's friends make of it, around a table, with beer. Done right, this gives it even more potency. It is storytelling squared.
A word of warning, though. The minute an author tells himself, "I'm going to explore what it is to be human" rather than just setting out to write an interesting story, or the minute a reader approaches a book thinking "This will give me profound insights", the magic quickly dissipates. The same is true of D&D. Be a philosopher; but amidst all your philosophy, be still a man - and there are orcs out there with stuff that needs taking.