Simple. Writing what you know gives authenticity to your words.
"But what about fiction?" the beginner asks.
She or he may cite all of the serial killer novels. If Thomas Harris had been limited to writing only what he knew, there never would have been Silence of the Lambs.
One way to write what you know is to do a lot of research. You can learn just about anything from books and the internet. You can write just about anything if you have the facts, but there's more to fiction than just having facts.
Fiction must be believable even if you're "just making the story up," to use a popular phrase we all use.
What gives fiction authenticity? It's not the facts and the research alone. It's the emotional truth you bring to your characters.
Believable fiction doesn't just come from getting the facts straight on your information plot. It comes from finding the underlying universal truth–the emotional truth–that is as real for an American woman like me as it is for a French accountant or a Japanese mother or whomever.
Believable fiction comes from the honesty and recognition of the emotions at work in the characters.
Writing what you know–the emotions you feel when hurt, scared, angry, lustful, or happy–is even more important in fiction because without that truth, your fiction will never succeed.
You might be able to fake expert knowledge in an information plot. You can never fake emotional truth and get away with it.
Be authentic is good advice to apply to your life as well as to the fiction you write.