They are essential for good security because they limit the scope and impact of a potential security breach by isolating different parts of the network or, in the case of network separation, completely separating networks and the devices on them from each other.
For example, by segmenting a network into different zones for different types of data or devices, a hacker who gains access to one zone will only automatically have access to some of the network’s sensitive information. With network separation, they will not reach the other network at all.