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.