There are two obvious answers: money and fame. These alternatives broadly structure Jon Reiner's recent piece at The Atlantic, in which he argues that the Internet has led to a decrease in opportunities for the first and an increase in opportunities for the second.
What's changed now is the payoff. The monetary rewards for writing are smaller than in the pre-Internet age. Even if every writing program in the country had a Zell grant to float the post-grads, there's no way that number of writers could enter the profession and sustain the day-to-day of eating and staying dry. But the psychic rewards, the seduction of an audience discovering you right now, have never been greater.
There's some truth to that, and it may well account for the boom in enrollment in writing courses that Reiner discusses. But I think the formulation of money vs. glory omits a third, possibly more powerful motivation: conversation.
To see what I'm talking about, look for a moment at two books that I happen to read recently. Both were written in those long ago days before the Internet: Nancy Friday's 1973 My Secret Garden and Patricia Wrede and Caroline Stevermer's 1988 Sorcery and Cecelia.
My Secret Garden is a famous—or perhaps infamous—collection of female sexual fantasies. Nancy Friday put advertisements in magazines and spoke to friends and acquaintances. Her book is mostly a faithful retelling of the rape fantasies, lesbian fantasies, masochistic fantasies, interracial fantasies, incest fantasies, bestiality fantasies, and other anonymous explicit sexual daydreams and desires communicated to her by her interviewees and interlocutors.
Obviously, Nancy Friday herself put the book together for both fame and fortune; it sold extremely well and became something of a feminist classic. The people who wrote to her, though—the ones who detailed their bestiality and rape and exhibitionist fantasies—received neither payment, nor (since they were anonymous) fame. All they got for their efforts, then, was a chance to communicate their thoughts to someone who said she cared about them, and a chance to participate in a scientific or sociological project. Participants throughout the book say that both of these motives—the chance to be heard, the chance to contribute—were important to them. But neither reduces easily to either money or fame.
Sorcery and Cecelia is a very different kind of book from My Secret Garden. Rather than feminist sociology, it's a romance mash-up—a romp that imagines the world of Jane Austen or Georgette Heyer with magic added. The book is constructed as a series of letters between two cousins... and authors Wrede and Stevermer each wrote one cousin's correspondence. The book, then, is a kind of game—and, indeed, in the afterword, the authors refer to it as a "letter game." They also make clear that the point of writing it, initially, was that it was a blast. That they were able to publish it eventually was a bonus—but the primary motivation was that they were enjoying themselves together.
In his essay, Reiner argues that "There have long been three kinds of writers: writers who write for readers (novelists, poets, memoirists, essayists, journalists, etc.); writers who write for other writers (students); and writers who write for themselves (diarists, shipwreck survivors)." The one group of writers conspicuously missing there is letter writers—who are both writers and readers, and who are writing not so much for someone else, as in order to get writing back. Not coincidentally, letter writing is a much closer analogy to what is happening in My Secret Garden and Sorcery and Cecelia—and it's also a much closer analogy to what happens with a lot of writing on the Internet.
As My Secret Garden and Sorcery and Cecelia show, these aren't new ways of using writing. Letters have long bound people together, as, for that matter, have diaries, which have very often (and contra Reiner's supposition) been semi-public documents, intended to be shared with friends and family. Indeed, if you weighed up all the reasons for writing on a per page basis, I'd guess that most people, most of the time, over most of history, have written from a personal desire to maintain bonds of friendship or form communities, rather than for money or fame.
The Internet hasn't changed that. What it has changed is the relative visibility and mixture of the different motivations. Letters used to be private—not so much, as it turns out, because the writers wanted privacy as because there simply was not the mechanism to make them more public. Now there is, and as a result, we are all reading everybody else's letters all the time. This is great for everyone who is writing in order to form connections or communities. It's not so great for those of us writing for fame and fortune, since all those letter writers tend to glut the market. But the fact remains that, for what most people want to use writing for most of the time, the Internet has undoubtedly transformed the world for the better.