 Kurczak po Chińsku (or Chinesse Chicken)   This is a very popular dish sold in Vietnamese restaurants in Poland.  Is this an authentic dish? (I guess not!) What is its real name?  
Even with sous vide, both overcooking and bacteria can still be a concern. In this case, it's about over-cooking.  Consider the meat just under the skin you want to brown.  If it starts at the ideal cooking temperature, by the time the skin is nicely browned in the oven, it will be overcooked.  By cooling it first the oven reheats it rather than overcooking it.  It's probably quite a thin layer that would overcook, but avoiding dry meat just under the skin is presumably why you're cooking it sous-vide in the first place. Chilling in cold water (ideally quickly) before putting it in the fridge or freezer reduces the amount the fridge warms up, protecting the rest of your food from loss of quality (or even spoilage if you put lots of hot stuff in there). Sous vide temperatures usually aren't enough to kill all bacteria, so even in a sealed container some can start regrowing if it sits around warm for a while.  Rapid chilling avoids that, especially for whole cuts of meats where the surface is the highest risk.  Then you can carry one cooling in the fridge if you're storing. 