Without the Assimilator, the Protoss would be unable to collect the precious vespene gas which vents out from almost every single planet in the warring sector. Built over escape holes for abundant gas, the Assimilator processes (it is unknown if it is with or without the help of Probes) the vapours into a usable substance which the Probes then collect and drop off to the nearest Nexus. Like the Nexus, the Assimilator does not require Pylon power to function.