The Processor is a Vaul building that harvests Vespene Gas.
As the name suggests, Vespene and Terrazine Gas are collected, stored in "honey-comb" containers (Protoss: Cube, Terran: Cylinder, and Zerg: Gas Bag) and refined in this building to be used in production and research purposes. Few biological units can infiltrate a Processor without succumbing to poisonous fumes with the structure. Due to their robotic nature, the Vaul felt little need to have safety protocols within the Processors toxic environment.
Before the modifications, the Xel'Naga primarily used robots in all their mining operations. Few ever personally entered a Processor seeing as the whole structure was automated. Most machines were designed with stronger and stronger alloys to withstand acids, toxins, extreme pressures, and temperatures. These robots were the prototypes for all Vaul robotics.