There are many reasons why someone would want to use tcpdump
You receive a bill from your cellular provider, and cannot believe how high the data usage is. You put on your detective hat, and pull out your toolbox - tcpdump. You launch tcpdump, and to your surprise you find that the weather application you installed was set incorrectly, and updating the weather every minute.
This is just one of hundreds of different scenarios which tcpdump can easily isolate and troubleshoot a high data usage issue. Just run it from the command prompt and instantly see a real-time snapshot of data traffic.
You download a calculator application which requests access to the internet. For what possible reasons would a calculator require access to the internet? You launch tcpdump, and to your relief, it is just checking for updates of the software from the developer's website.
See what is running behind the scenes with tcpdump. Most of the times, it is what you would expect, but sometimes, there may be something fishy going on.
You are developing an application which connects to the internet using tcp socket controls. It is not working as you would suspect. You launch tcpdump, only to find out you are pointing to a non-existent URL.
tcpdump was compiled from the same source as tcpdump on Linux systems. There is no better command line tool for capturing and debugging ip traffic. If you are already familiar with tcpdump on linux systems, then you are already all set to go with the Android version of this tool.
You are on your home wifi network, and cannot access file sharing on one of your computers. Is it a problem with your android device, or your home computer? You can run tcpdump to help narrow down where the connectivity problem is occurring.
Sure. It's fun to learn to use a new tool. The transport protocol for the internet is TCP/IP. Learn more about how the internet works by inspecting packets.
These are just a few examples and reasons of why to use tcpdump. Because Android devices can multitask, you can setup tcpdump in one window, and run applications as you normally do. As a developer, and a network troubleshooter, it is one of the most valuable tools one can have in your utility belt. You can capture packets with various switches, and log them to a file. Once you finished your capture, you can export the file to a PC and run debugging tools against the trace (Wireshark for example).