"Write what you know" is the most basic advice given to amateur writers for a reason. When you write what you know, you have to do less research and are less likely to get things wrong, not to mention that it's far easier for those who are just getting their toes wet writing. It's easier to come up with a plot when you're working with what you deal with in your everyday life because you know what's possible, and what's interesting to other people who are interested in the same things you are.
Writers writing what they know can lead to greater depth and quality. For instance, when I read about people who are disabled, it's easy to see which writers know about being disabled from personal experience, and which ones merely think it makes a good plot point. I can tell which writers have been/are in the military, when they're writing about the military. It's not just trivia that makes the writers who know what they're talking about on an intimate level stand out. You can throw trivia at me all day, and I'll know the difference between that and somebody who has experience, even if I myself have no experience with what you're writing about. If you're an insurance adjuster writing about an insurance adjuster who has whacky adventures, you're going to have a more accurate perspective on that than somebody who googled it. This doesn't mean you can't stretch yourself- if you were in a major earthquake in California, you can likely write convincingly about a major earthquake in the fictional world you've created.
When you write about subjects that you don't know, you run the risk of getting them wrong. We've all watched movies that lay in our area of interest and got everything completely wrong- you think Hackers was anything near accurate in regards to computers or hacking? When you write about subjects you do know, you get them right unless you deliberately decide to get them wrong.