Wireshark should be a required tool for any developer writing netcode. If you don't understand the OSI model, or how a 3-way handshake works, how can you possibly debug your code?

I've had to work with customers before who were writing code to send data over XML to a web server before who had no idea how to use tcpdump and Wireshark - it was unbelievable, but I was able to show them bugs in their code because I could see where the TCP conversation broke down.