Blue hose:
It allows for air to enter the secondary pipe. This prevents it from becoming a full siphon. It allows for a trickle to flow down the pipe. You only want the secondary to become a full siphon when the main become blocked for some reason, so it can handle the drain volume.
If the secondary were to become a full siphon and the main is not blocked you would get an instant draining of the overflow. Then a woshing sound as both the main and secondary suck air. Then the main and secondary will go back to normal until that blue pipe gets covered with water again. Repeat over and over again.
Because your pipes are not in the overflow box you need that blue hose. My pipes are in the overflow box. Its an external box. I made my overflow box deeper and my secondary has a hole in the top without the blue hose stuck in it. Should the main get stopped up, water will cover this hole before the overflow fills up to a point where the emergency takes on water.
