Keeping a copy of photo ID

[Stephen Lassiter]

I am renting out an apartment in my backyard. My applicant's co-signer is questioning my need to keep a copy of their photo ID. Everything I read on Bigger Pockets says I should get a copy of the applicant's photo ID. My purpose for requesting photo ID is to verify the identity of the applicant. But I want to ask everyone what are their reasons for asking for ID? Specifically, why do you keep a copy once you've verified the applicant's identity? How long do you keep a copy of the photo ID?