The assimilator is a protoss building used for harvesting vespene gas. Although materials were usually warped in from Aiur, vespene is still required for these materials to be produced and the creation of warp rifts.
Tal'darim assimilators wretch vespene gas from the ground, triggering earthquakes and decimating surrounding ecosystems.
The following section contains information from a previous version of StarCraft II which is no longer valid.
The assimilator's StarCraft II form went through a few revisions before the designers decided upon the current version in-game. The intention for the update was to showcase the functionality a little better by actually seeing the little gas cubes being manufactured inside. The final form is effectively a dome that covers a vespene geyser, with a crater visible inside.