Al,

A very reasonable question.  The mini DIN-6 configuration is the amateur radio version of an analog radio interface.  It contains key signals.  PTT (to radio), COR (from Radio), AF Audio Out (from Radio), Modulator In (to Radio), Discriminator Out (from radio), and a common ground.   It was standardized for interconnection of external modems (including TNCs), as you say, decades ago.

It was adopted for the DRAWS™ HAT so that a straight through cable with male mini DIN-6 plugs would make an easy radio interface for the audio/modem + PTT/COR connection.  If a radio has the same signals but on a different connector then a cable must be made or purchased with proper wiring.

This generation of connection did not include radio control (beyond PTT).

While the mini-DIN 6 comes very close to a standard, not so for rig control with each manufacturer implementing their own version of CAT/CI-V  some use serial ports, some USB ports, some other data jack.

To build a general interface, NW Digital Radio took what would reach the most radios.  USB connections for rig control (and audio if the radio supports it) plus 2 mini DIN-6 connections for audio and COR/PTT.   I personally prefer the hardware PTT of the GPIO connection through the mini DIN-6, when the application supports it, vs CAT control of an unknown baud rate and interface.  Many VHF/UHF radios don't even have rig control and simply use the mini DIN-6.