Generally speaking, employers are required to submit copies of the W-2s to the IRS as well as the employee, so the IRS can match their records (tax withholding payments, etc.).
If your employer submitted their W-2s to the IRS late, or not at all, or the amount reported was different than what you entered on your tax return, you may be required to send your W-2 copy as proof of your wages and withholding.
