When the IRS mandates an EIN, you don't have a choice. But here's a key reason why it's a good idea to use an EIN for business even when it's not required: avoiding identity theft.An EIN helps keep your personal and business finances separate. It reserves your Social Security Number for personal needs and keeps identity thieves from ripping off your personal assets. The IRS recommends using your Social Security Number only if you have a very simple business structure and no employees.