The purpose of a network diagram in IT planning is to visually represent the layout and structure of a network's components. This includes hardware like routers, switches, servers, and the connections between them. Network diagrams help in understanding the flow of data, identifying potential bottlenecks, and planning for expansion or upgrades. They serve as essential tools for troubleshooting network issues, conducting audits, and training staff on network configurations. By providing a clear depiction of the network architecture, these diagrams enhance communication among stakeholders and facilitate efficient IT management.