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.