The USB port on Android phones is very useful for creating devices that connect to the phone and are also powered by the phone. One major advantage over Bluetooth is that Batteries are NOT needed and you don't need to worry about the privacy/security vulnerabilities of Bluetooth. This video demonstrates how to connect an electronic circuit board -- such as an Arduino or Raspberry Pi or a custom PCB -- to an Android phone, using simple off the shelf adapters and cables. The method shown is very handy for development and debugging. But when you are ready to build an actual product or nicer prototype, these connectors and adapters are not required because this can be built in a very compact way with just a small circuit board. The serial device in this video is a Raspberry Pi Zero, which was programmed to output serial data packets over its standard serial UART pins.