The audits also catch a lot of cases where people don't know how to file anything beyond their basic employer withholdings. If a poorer, less educated person gets a 1099 for a side job or does an early withdrawal from their IRA, they are likely to fail to report it properly, or pay the required taxes. This is not usually intentional, they just don't know better. Even if they know they have to pay taxes on the money, they don't realize that the IRS didn't already take their cut just like they do from a paycheck.