Understanding the functions and inter-dependencies of the layers in the OSI Model is critical to network troubleshooting. Since the original OSI model was created, the telecommunications industry consolidated around the IETF standards built on the TCP/IP stack.

Network engineers typically focus on layers 1-3 (through IP), while application developers understand their application behavior and that leaves TCP in a gap.

TCP plays not only a critical role in reliable delivery of packets but also impacts performance at least as much as "the network". TCP runs above the network and so falls into a zone of non-understanding. If a network engineer really wants to help solve application performance problems then TCP knowledge is needed.