Short answer: Because water and concentrate do not mix in the feed pipes (post-mix), the 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 physical separation of the water and concentrate flow paths. 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 and liquid-line cleaning is still required.
Why does it matter?
Diluted sugary liquid sitting in pipes is the strongest driver of biofilm. In this system, because the concentrate is carried in a separate line, the diluted product is prevented from sitting in the line; the water line stays nutrient-free too.