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.

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

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