I’m pretty sure you’re required to provide your driver’s license info at some point during the sign-up process, actually. Though it’s been a while, so I don’t remember for sure.

If it’s like I remember, it’s to confirm that the person on the ID matches the person who’s signing up. Banks do the same when the teller asks for your ID. And so do the people who ask for your ID when you go to vote. It’s the same basic process. It’s just digital instead of in-person.