The Assimilator is the gas extraction building for Protoss. When constructed on top of a Vespene Gas Geyser it allows the Protoss player to collect Vespene Gas by sending Probes into it. Probes collect eight gas at a time and return it to the Nexus until the geyser is mined out when they only collect two gas each time. Assimilators can only be built on Vespene Gas Geysers and are the only way a Protoss player can mine Vespene Gas.
Since the Assimilator builds on its own, it can be used to Gas Steal the opponent, disrupting Terran production. This strategy is strong against Terran because all Terran units that can effectively kill Protoss units require Vespene Gas. Since the Terran is forced to delay the construction of its Factory, the Protoss player can seize the opportunity to attack the Terran while he does not have Seige Tanks.
The Protoss player will usually warp in the Assimilator very early in this match-up on to enable Dragoon production.
Similar to Protoss versus Terran, the Assimilator can be used to Gas Steal the Protoss opponent in the same fashion. This strategy is especially viable when the Protoss player opts for an aggressive 2 Gateway build order against the opponent because the opponent will not have Vespene Gas. In this scenario, the opponent would be forced to defend with tactics such as using a Shield Battery and/or a second Gateway to fend off the Protoss player's Zealots instead of teching to Dragoons.
The Protoss player rarely uses an offensive Gas Steal strategy with the Zerg because Vespene Gas is not as vital to early Zerg production. Further, the early presence of Zerglings can usually fend off an Assimilator easily.
The Protoss player will usually warp in the Assimilator later than in the PvT match-up as the Protoss does not need Vespene Gas early on as it seals off the natural in the common Fast Expand build order.