Simple amplitude control
When you're changing the amplitude of a sound, if the amplitude is changed very suddenly and significantly, it may create a sudden discontinuity in the waveform which will be heard as a high-frequency click. (That phenomenon is demonstrated in the example "Click resulting from amplitude change".) To avoid that problem, it's necessary to interpolate sample-by-sample from the old amplitude value to the new value, over some small period of time, to create a slightly more gentle fade to the new amplitude factor.