It might be a niche case, but it's really frustrating using the various sleep-related audio apps at night with earphones and not being able to select a minimum volume that is both loud enough to hear but not so loud it keeps you awake (example: Audible with a sleep timer).
I'm sure there are other uses for the finer control as well.
I had the impression the problem was physical limitations with the dial. With fully digital controls, it seems like this wouldn't be an issue. But obviously you need more than the handful of steps the volume buttons on the side of the phone give you.
I don't know if there are any programmatic system-wide ways to do the same (every app can implement it in their software mixer though). I haven't had great experience with third-party solutions that obviously cannot integrate with the OS at the same level.
Have you tried pestering Audible? Would probably take a while, but it feels like the kind of thing that would be hard to "un-justify" once you started arguing for it.
Obviously any major app supplier may be reductant to invest significant dev time to offer a workaround for an OS limitation (and Audible is just an example), but yes it's worth raising I suppose, thanks.
I'm not even sure if it would be straightforward with a rooted device either.