This unit is exceptionally useful.
As a straight forward mixer, you can mix up to four signals and route them to four separate outputs, and each output can be set independently to sum the signals in a unipolar (positive voltage only) or bi-polar (positive and negative) fashion. Leaving the first input unplugged also allows you to add an offset to each output via the top row of knobs - very useful!
Where this unit comes into its own however is as a sequencer - you can create four separate sequences from the same input voltages - which can then be used to drive four oscillators to give some nice chords, or if you add an A-151 sequential switch (recommended!) you can create much longer and more complex sequences by switching between the outputs and sending them to one oscillator.
You can also mix LFOs / envelopes etc to create interesting and evolving modulation sources.
You can even (very carefully!) feed the outputs back into the mixer and use it for 'zero-input-mixer' music (disclaimer - feedback is tricky to control! Be careful!)
The usual high Doepfer standards apply here, the perfect blend of affordability and usability.