One of the key controls in cyber world is the ingress/egress points to the network. Without sufficient control, threat actors are able to penetrate inside causing system or service disruption anywhere anytime. On top of network aspect, controlling of physical access to the equipment is also important.
In physical world, establishing physical perimeter is far more challenging than that in the cyber world. Three are “proper” means to reach a region and multiple “improper” means to do the same. Effective control is proper policy for “illegal” entry.