This interfaces with four parts of a handheld radio transceiver:

* 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.

The other element of this is CAT control, which is a serial protocol for tuning the radio (among other capabilities); CAT control for tuning + transmission control for turning on/off transmit + audio abilities (to send and receive) basically makes a radio 100% digitally controllable, which can be helpful for everything from voice traffic to data mesh to scanning/monitoring/etc.

The radios that this is meant to work with don't have a real CAT control interface. Some can do PTT through this interface but some don't, my Baofeng requires you to do VOX mode when using an APRS cable. CAT allows for way more control than just PTT. You can't change frequency over the serial port unless you are reprogramming the VFO presets every time which takes a relatively long time because you have to reprogram the entire memory. Most HTs don't support CAT because HTs usually are meant to be used in the field where they are programmed ahead of time. Even my super fancy FT2DR (and the newer versions) doesn't have control over serial.