Knobs and sliders allow users to leverage their spatial and manipulation abilities to specify a value from a continuous range.

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.

To add to this, the precise value of a control isn’t relevant in most mixing and sound design operations, only it’s value-within-range. Most of the decision-making work is done by ear, not by number. The knobs (and sliders) give you a quick reference to “can I get a bit more out of this”.

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.

Very relevant point. The end result of any individual sound is entirely dependant upon the other sounds accompanying it within the same physical space.

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.

They also serve as a kind of memetic device. To see the current state of things, the user does not have to (1) read a bunch of numbers (2) remember the current state. Instead, they can look at the image (full of knobs and sliders), and utilize the much greater visual processing capacity of the human brain.

Unlike a slider, a GUI knob's visual feedback and skeuomorphic comfort comes at the cost of ergonomic interactivity.

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" :)

two words: scroll wheel

more words: scroll wheel with disengagable "clutch"

The only alternative UI for many of these knobs is a linear gauge/slider. A knob is often modulated and automated so it's not just an input widget, it's also a gauge. It needs to convey an animated value to the user.

Typing is usually out of the question since you need to do it by ear. Some controls obviously have numberic input in combination with the knobs, for extra flexibility (e.g. if you made some notes with settings and just want to enter them).

Many of the values use mappings that make little sense as numbers. E.g. a knob marked 0..10 could mean a filter cutoff frequency from 20 to the nyquist frequency, logarithmically. Entering values in a text box with such arbitrary limits is a terrible experience (is it 20...22000...or 20..48000? or 0..1? or 0..100%?). Knobs hide the complexity of the logarithmic mapping and it's easy to to quickly set a value by ear.

In a few cases the knobs are decorative and could just as well have been e.g. most fixed step knobs could be combo boxes. For example a vintage synth often had a rotary knob to select between 3 options (waveform, for example) so now you can see that in ui's too. Doing that in a computer interface is purely for visual appeal.

It is not only the analogy. I like to use Midi controllers which have hardware knobs mapped to software knobs for live performances.

Sure, you could map a knob to a slider or anything really, but it makes the interaction easier

Sliders are a great fit for a touchscreen. And knobs are basically sliders that take up less room.

I don’t see the problem? If you give me a numeric input that doesn’t allow me to drag or swipe to change values then I’ll swiftly be looking for a different app.

What's a better UI for this? The times when a knob (or slider) are appropriate is when you're adjusting something by eye or ear (e.g., you don't know the exact destination value in advance).

Unlike a number box, they show the range of potential values and contextualize the current value - similar to a slider but more often those are linear when knobs are non-linear. Just my 2¢

It is just skeuomorphism but sometimes musicians bind those knobs to real midi instruments.

Every serious musician I know has a MIDI controller on their desk, each mapped to specific controls on their DAW. A physical knob that maps to its GUI equivalent is really helpful.

This skeuomorphism isn't without cause.

Knobs are just sliders using less space.

You need knobs and sliders to convey information visually.