Comment by FroshKiller

It's a simple prescriptive rule in English. If you are writing about a small number, like less than ten, spell it out. For example: "According to a survey, nine out of ten people agree."

But if you are writing about a large number, particularly one with a lot of different digits, prefer writing the digits: "A mile is 5,280 feet." Compare that to: "A mile is five thousand, two hundred, and eighty feet."