It allows for dense controls and everyone's used to them. I don't find them to be a problem, they aren't intuitive in that you might think you're supposed to grab the knob and "turn" it with a circular cursor motion or something, but once you learn to drag linearly, they're an easy to use and consistent interface. And as giancarlostoro mentioned, you can map them to a MIDI device if you want to twiddle knobs while playing/recording live.

I'll add in addition - the skeumorphism here is generally pretty functional, you touched on this when you said "everyone is used to them"

But the layout of these buttons, while certainly not standard, is generally familiar across various filters, etc. So if you are dealing with a complex interface the skeumorphism absolutely helps to make the input more familiar and easily accessible.

This is what skeumorphism is for and this is a great place to use it.

Imagine if the symbols for "play" "pause" and "stop" were changed simply because it no longer made sense to follow the conventions of a VCR, then multiply that by an order of magnitude.