How does the phone number help?

Never received a phone call at 5am on a Sunday because a bug is causing a valued customer to lose $10k/minute, and by the way, the SVP is also on the line? Lucky bastard

That's the polite version of "we know where you live". Telling someone you have their phone number is a way of saying "we'll call you and expect immediacy if you break something."

Wanna be treated like an adult? Cool. You'll also be held accountable like an adult.

Presumably they will be contacted if there's a problem. So the hire has an interest in not creating problems.