Feedback.
My main use of additional IO.
In fact you can not go Audio out and CV out into audio input.
Crazy experiments in this field.
Check out the CV kick:
https://www.elektronauts.com/search?context=topic&context_id=228178&q=%23elektron-gear%3Aanalog-four%20cv%20kick%20&skip_context=true
CV out to CV in:
Another cool thing is to drive some (semi-) modular with the midi and CV outs, and treat it’s output within the A4.
This way, you can even use tubes LFOs + Keytrack on A4 filters.