The big empty bulb up top provides a large, relatively cool surface for the mercury vapor to condense on.
When one of these is in operation, mercury is constantly dripping down the sides of the bulb, and running back into the pool at the bottom., It is then heated by the arc, turned into vapor, and rises up into the bulb to condense again. This cycle maintains a relatively constant vapor pressure in the tube, which is very important to avoid arcbacks and flashovers.