They actually don't have all the information they need in every case, as not everything is reported to the IRS. But yeah, they already have the majority of info

They actually don't have all the information they need in every case, as not everything is reported to the IRS. But yeah, they already have the majority of information, particularly the most impactful information, and for the majority of people, all of the information that they need. Anyhow, I'd imagine in any case it is still useful as a way to validate information.