Even if you don't make $600 per year they need to track it and send totals paid out to the IRS. They would need the documentation in case the IRS came knocking on their door about their basis for the totals.
It also is used to show that one person doesn't have multiple accounts to avoid rotation issues.
I am not sure what exactly they do to verify, but I know from some companies, I have received error messages that they are unable to verify my SSN.