Hello you are so kind to answer my newbie question! So I got an answer from Arturia Support and they told me:
"If you're using a mono input on the sound card you would need to use the left one, which sums everything to mono,
You would not lose anything else than some stereo information in the case of using reverbs and delays which are stereo,
In your use case I would simply recommend using delay or reverbs after the unit in your daw, to get a better stereo image"
Anyway they didn't mention the setting we were discussing:
Utility/Audio/Stereo to Mono => On
Utility/Audio/Stereo to Mono => Off
So I asked them again, I'm waiting for the answer.