"Write what you know" is some standard advice to new writers. I came across it again in some blog this morning and I stopped to think about it. Why is it good advice? From what I've read it means to say that write what you know about because you know about it and therefore will write a more realistic story.
Let me take a different slant. Maybe it means to write about what you know because that is a subject in which you are interested. I'm know a lot about science (ten years of college) so do I write SF because of that or because I spent that much time studying science because I'm interested in it.
Writing is hard, especially when you start and don't get much positive feedback, rejections don't count. So if I wrote historical fiction about the civil war would I be able to stick with it long enough to get published? I doubt it. But by writing science fiction I can explore past the bleeding edge of science and technology.
The same goes for other occupations. There is always a push to get students interested in fields where pundits think the USA is falling behind. Would you go to a doctor who said that he got in the field only for the money? Would you drive over a bridge built by an engineer with that attitude? Or would you prefer a bridge built by someone who as a kid put together the ramp from which other kids jumped their bikes?
I suppose it comes down to passion. It's best to follow your passion as close as you can and still make a living. Or be born rich.
I guess I was feeling philosophic today.