* Audio In (mic)

* Audio Out (speaker)

* Push To Talk button

* Serial UART reprogramming interface.

It's useful because you can hook your computer up to a radio with a single device and use the cable both to reconfigure the radio but also for APRS (Automated Packet Reporting System), which is a really cool system by which standard digital packets containing arbitrary information are repeated through a network of ground stations worldwide. Think an "IOT mesh network" from before buzzword technology (and unfortunately with a much higher power requirement than the modern LoRa based systems).

This has always been possible using a sound card and some method to manipulate the PTT interface, and a separate UART cable swapped in for reprogramming, but having everything together on one cable is a nice convenience.