The electronic transmission from employers actually goes to the Social Security Administration. It takes months for the IRS to get the information and match it up to the employees' tax returns. (Remember that the IRS is using 1960s-era computer technology.) When you e-file, the W-2 information that you entered in the software is sent along with your tax return. So whether you e-file or paper file, the IRS gets a copy of your W-2 with your tax return. This lets them do at least some preliminary checking without waiting until the transmission from the employer is available.