Comment by Ashanmaril

I don't know, I've used these and I think they're actually kind of ingenious. Perhaps not the most intuitive thing, but once you understand how it works, it's pretty easy to use, and having the entire vertical area you can move your mouse for granularity is probably superior to moving a circle back and forth on a tiny bar.

I imagine this was originally implemented just because they wanted the UI to be a 1-to-1 matching of a soundboard, but it actually has the advantage of what I mentioned (granularity), as well as being able to display the level within a circle, AND you can fit a ton of them on screen without taking up much space. They're actually very functional.