Not only does it help prevent things from happening that could be a problem later, it helps prevent even accusations of such, if it's well known that the person involved holds to this rule and expects others to hold them to it as well. Say what you will about Pence, love him or hate him, nobody is accusing him of sexual misconduct - and that's the point of the rule.

[0]: https://en.wikipedia.org/wiki/Billy_Graham_rule