An increasing number of sophisticated, high-performance security and content-aware devices are at layers 4 through 7. This calls for, among other things, even greater sophistication of network emulation from test equipment. When Layer 4-7 devices are not properly tested, they face a greater risk for failure within production networks. Failure of these devices opens the network to the threats they were made to protect against.
Sophisticated test emulation means replicating the exact behaviors and traffic between devices based on stateful interactions. You do not achieve realism if stateful devices simply receive random traffic with dummy payloads; they must carry on a conversation that has specific meaning from beginning to end.