If you make program with a bunch of different exercises, and are modifying your program a lot, it can be a pain to keep track of what your next exercise should be.
If you are automatically tracking reps, it can just announce whatever the next thing in your program is, and start rest timers automatically...
It's not really about tracking reps as much as tracking where you are in a program that it is useful for...
It never occurred to me that I'd want to automatically track reps. I mean, it could be kinda useful, but I'm also taking 45 to 60 seconds cool-down between exercises anyway, so it's not like it gets in the way.
That also moves the app onto the next exercise in the programme. It's manual, but it's not intrusive and can't be wrong, like a fitness tracker, that definitely can be. So shrug, just personally, auto-tracking reps isn't something I need and wouldn't be a big selling point for a band/watch over a ring.
Even if it is just for swiping or pressing buttons to switch between exercises in your program. But there are other uses for accelerometers.