Why is segmentation recommended for network design? that is, what makes it better to have a separate network segment for web servers, one for the back-end office processing, one for testing new code, and one for system management?

Segmentation is recommended for network design because it enforces physical separation, where test code could bring down the whole corporate network, which would cause far more damage and chaos than just crashing the testing LAN if there was no physical separation. Example scenario would be: If the testing network is completely isolated, it is not possible for a piece of test code to accidentally affect the management LAN or back-end processing LAN. It might affect the whole testing network, but it will be isolated to that one segment.