I’m total noob when it comes to CV–but for one you can use all the advantages of the sequencer on the A4 basically. Right now I’ve got mine just setup to play notes (Pitch/Gate) so that takes up CV A/B. I’m using CV D/C just to control the filter cutoff of each filter–but you can p-lock all these parameters.
In the case of the MS-20 mini this is something you can’t do (I might be wrong here) with MIDI CC. I’m pretty sure it only receives note data over MIDI. But you can P-lock and control basically any parameter with a CV input on the MS-20 with the CV track of the A4. You can also use the LFOs on the A4 to control, again, any parameter with a CV input.
There are lots of youtube videos of people sequencing other synths with the A4 via CV, you might have to dig to find something really cool though. Dataline did just this little album with the MS20 and A4
Maybe it is just something to think about for the future (or not at all). I just find it fascinating and fun and I’m getting sounds out the MS-20 that would be hard to get otherwise because of its comparatively limited functionality to other synths that have more modulation options.