So why does Apple need to use yet another connector? What's wrong with USB-C or Lightning?
It comes down to two things.
First is size. The UAC port is thinner and about half the width of a USC-C or Lightning port, so it's better suited to smaller accessories such as headphones.
The other factor is that UAC relies on standard USB protocols, which means that apart from a different port and cable, there's no need for any design changes or authentication chips and such.
So, the bottom line is that if you purchase an accessory that makes use of UAC then you will have another cable to contend with, and you might find yourself having to buy new cables or mess about with dongles if you lose or misplace the cable.