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Comparators output a logical "true" (high voltage) when one signal is above a reference signal. On this device, rather than a logical true/false, a continuous Pythagorean distance is sent to the output which means that you can use it for processes which you may wish to modulate based on how "far" two signals are apart (based on Pythag. distance, which is different than subtracting the two signals) or as an audio effect for a pair of oscillators that works a bit like a ring modulator only much more harmonious.
As an audio effect, it merges two signals into a richer and more complex tone that interacts more deeply than typical summing mixers. You can produce harmonics, organic variances, and saturation-like waveshaping. Be sure to use both inputs though, with two different signals (otherwise, there's no effect at all).
The "driver" is the primary signal and the "passenger" input is used as reference for comparison. The balance between the two can be controlled via the large central dial for a range of intensities and sounds. The bottom dial offers a continuous range of Pythagorean "methods" and can similarly be tuned for a range of tones. Both of these knobs are cv controllable and have a joint set of attenuation and bias controls. At the main output (top right), a gain control lets you account for any volume changes.
If you're looking for a way to more organically combine cv signals (vs just summing) or for a more interesting way to mix a pair of oscillators, the Pythagorean Nonlogic Comparator is there for your experimentation...