This logically incoherent argument often arises in situations where people have an assumption that is very ingrained, and therefore taken in their minds as a given. Circular reasoning is bad mostly because it’s not very good.
Example: The word of Zorbo the Great is flawless and perfect. We know this because it says so in The Great and Infallible Book of Zorbo’s Best and Most Truest Things that are Definitely True and Should Not Ever Be Questioned.
Tomorrow: appeal to nature (source: Skeptical Inquirer, July 2012)