Virtual knobs (and sliders) invite experimentation where a numeric control or text box doesn’t (so much). You can play with them. The dragging action gives you a little bit of tactile feedback.
Just as a file isn't only a file, but an entity in a filesystem, a sound is not just a sound, but an entity in a sound environment.
The whole point of physical knobs is to leverage a person's thumb and forefinger (or tennis muscle if it's a huge knob) to twist precisely and efficiently to a desired position. With the two most popular digital input devices that torque doesn't exist-- you can't "twist" a mouse or tweak a touchscreen to change the position of a knob.
At least in most of the audio GUIs I've seen that use knobs, the author seems to have no idea that their skeuomorphic interface actually degrades interactivity. E.g., if they had simply used a slider the user could swipe their finger efficiently to quickly change several sliders in a row.
Even worse-- with a knob you have a similar "90-degree ambiguity" problem as with touchpad scrolling. Do you use dx, dy, or some other algo to choose the relationship between touch/drag and knob rotation?
Even worst, some devs try to "bring the torque back" by coupling the knob movement to the distance of the mouse from the center of the knob. I really wish someone would feed this awful behavior back into the physical world, and ship a mixing board with knobs that only update their value if you twist them using an included set of "knob tongs" :)
more words: scroll wheel with disengagable "clutch"
You could also ask users to just type in a number, but that interrupts the flow and pushes some other data out of their short-term memory. Composers and audio engineers would probably prefer to keep that part of their brain free to focus on other things.
Sliders can also be laid out in an array (as on a mixing console) to give at a glance a graphical representation of relative levels. Another example is on the classic "two turntable" DJ mixing setup where a horizontally-oriented slider is used to control the balance between left and right.
Knobs' advantage is they are particularly space-efficient.
So you tend to see sliders used for the primary controls in situations where its useful to compare them side-by-side, and everything else as a knob.