These flood and drain fittings are used with flood and drain trays to create a passage for the nutrient solution to flood into the systems and then drain and also for an overflow.
The 13mm fitting is usually used as the inlet and is attatched to a pump to flood the system, once the pump turns off it will drain back through the pump and into the tank.
The 19mm fitting is used as the overflow with a 35mm spacer and top screen. This allows any excess nutrient solution that is pumped up from the tank to fall back into the tank through the overflow to prevent it flooding the tray and your floor.