An open, non-segmented network is a wide-open door to cyber attacks.
If a hacker finds a vulnerable access point and manages to exploit it, he can literally raid your environment. It can also use that point as a gateway to access a larger part of the network and potentially any resources connected to it - product designs, recipes, machine controls, financial data.
And it is not only external threats that pose a danger to unpartitioned networks. There are also internal threats, e.g. dissatisfied employees or human errors (including incorrect system changes) that can cause serious damage if not controlled by a protected perimeter or other forms of access restriction.
That's why network segmentation should be part of any company's industrial security strategy: think about the devices you use in your business, what function they have, what sensitive data is on them and who is using them, and divide them up into different networks that can only be accessed with secure passwords.