I have a Ph.D. in the history of religion - and until now I never did anything with it. I ended up working in the tech sector, but I have all this arcane knowledge floating around in my head. Every now and then I would completely overwhelm a conversation partner who happened to ask me about religion or mythology, but otherwise I didn't find much of a use for the stuff I learned in school. Then one day I was reading a Dan Brown novel, and something clicked: I realized that the interesting stuff that Brown uses to spice up his thrillers is very much like what I learned all those years ago. And so I thought to myself: "I should try to write something like that, and see if people like it."
Most likely my dissertation supervisor would be spinning in her grave if she wasn't still alive, but I'm having fun and so far my readers seem to be having fun, too. As long as people like my books, I'll keep writing them.