It is, and it kind of sucks. Worse yet: in different VST plugins, for instance, sometimes you're supposed to click the knob and drag up and down, sometimes you're supposed to move the mouse in a circle as if you were rotating the knob.

Yep,every VST seems to be a learning exercise. But the variety of layouts and colours can also be inspiring. That said, the most garish were rarely keepers.

Was going to say. It saves screen space - should you need to save screen space - while allowing fine control. A slider would take much more screen area.

This is a bigger issue on synth VSTs that have tens or hundreds of controls. When screen resolutions were smaller it was a toss-up between hybrid vertical/rotational scrolling, horizontal/vertical window scrollbars to get the controls to appear at all, and multipage UIs.

Some designs, like Korg's MS20 VST, had all of the above.

You could keep the vertical "out-of-box" dragging but use a little vertical bar to show the current level.

Humans are terrible at reading angles quickly.

Humans may be terrible at reading pie charts quickly, but I think we do fine with visualizations such as clocks, gauges and circular controls. I wouldn't even be surprised if a circular gauge can be read more quickly or with less attention than a linear one (size/area being equal).

When staring at a mixer with 60 or more knobs you can see the difference in reading speed very quickly:

◐◑◒◓◐◐◒◒◓◐◓◓◐◐◑◐◒◓◓◐◐◑◒◓

(oops, HN is not showing the unicode rectangles but you can imagine that they look like a barchart and you can easily spot the highest and lowest)

It's to replicate audio disks - missing the opportunity that a different interface requires different interaction mechanisms.