The oscilloscope is the original audio visual: a beam sweeping left to right, deflected up and down by the signal, drawing the waveform as it happens. Synthesists watch one to see what a filter actually does; engineers to catch clipping as flat-topped waves.
In X-Y mode the sweep is replaced by a second signal and the scope draws Lissajous figures, the basis of an entire genre of oscilloscope music and art.
The phosphor-green trace is deep in the visual language of electronic music by now. Photism’s signal scene includes a scope mode for that reason: the instrument’s honesty, rendered in the show’s palette.