Ahh, to type the very last word of a 400-page novel. There's no feeling quite like it. Well, except for the feeling of typing the last word of a 401-page novel, but now we're just getting into semantics.
You can change the world with your writing. It doesn't happen all the time, but it is possible. Not likely if you're writing catchy slogans for oatmeal cookies, but if you are writing about anything substantial, there is a chance for your words to truly make a difference. Think about how much the concept of "Big Brother" has become a part of the social consciousness ever since George Orwell wrote 1984, and how that has shaped our views of government interference in our lives. The writings of Frederick Douglass, a former slave, helped push forward the abolitionist movement, and his three autobiographies shed a lot of light on what slaves went through, helping to change the popular view in that regard as well. And how empty would our world be if Stephenie Meyer had never written Twilight?