The theoretical purpose of auditors is to allow companies to prove things to the public, without proving them publicly. That might be a company proving to its

The theoretical purpose of auditors is to allow companies to prove things to the public, without proving them publicly. That might be a company proving to its shareholders that their accounts are in order, without publicly releasing every last deal and detail. If the accounts say Google's got a billion dollars in the bank, investors having someone check they have a bank account with a billion dollars in is basic common sense! The practical purpose of auditors is a lot less clear, because companies get to choose their own auditors, and nobody hires auditors with a reputation for being difficult...