Security is about deciding what's forbidden, what's allowed and enforcing these decisions.
There's the technical part, doing what you can to technically enforce the decisions.
And there's the human part, managing things in a way that reduces related uncertainties.
The human part is the most important. You cannot enforce every decision technically. Besides, you have to allow for people to switch on/off certain features, eg to allow for good functioning outside of the company's premises. So, you forcibly leave some room for employees to decide by themselves whether they respect the rules or not. And that's the moment when the human factor matters.
The most important tool for the human factor is the security policy. You have to say what you do in terms of security and to do precisely what you say.
If you don't do what you say, you invite people to force the system, they think they can fool you. It may go as far as disregarding completely the policy.
If you don't say what you do, you invite people to rebel and contest the security measures. Additionally, you scramble people's understanding of your security policies, which may lead them to give up trying to respect it.