Short answer: Because water and concentrate do not mix in the feed pipes (post-mix), diluted product does not sit in the lines; this reduces the risk of biofilm and spoilage — it is risk-reducing, and regular cleaning is still required.
Two separate outlets provide a built-in air gap (backflow prevention). When the concentrate is drawn from a closed bag-in-box, air contact is also minimised.
An honest note: This is a risk-reducing architecture, not "sterile"; regular reservoir/line cleaning (CIP) is still required.
Why does it matter?
Diluted sugary liquid sitting in the pipes is the strongest driver of biofilm. In this system, diluted product never sits in the lines; the water line stays nutrient-free and the concentrate line stays low-water-activity.