Network segmentation is considered one of the silver bullets that could have prevented many of the high profile breaches.
One of the important ways to minimize the exposure to attacks is to segment your network so that critical production servers are separate from the rest of the servers, which includes Dev, QA, and staging environments. In simple terms, network segmentation is a process of splitting the network into multiple subnetworks. System administrators can then apply individual control over each of these subnetworks. Doing this brings a lot of benefits, which includes major security benefits.
Because of the advantages that a properly segmented network brings, PCI DSS environments, having network segmentation, are a mandatory thing to follow without which you cannot pass the compliance.