The Ultralisk Cavern is a late-game Tech building created by Zerg for the purpose of allowing the production of Ultralisks. Ultralisks are produced at the Hive, as the Ultralisk Cavern does not build any units.
Additionally, the Ultralisk Cavern allows research for two upgrades to the Ultralisk: Anabolic Synthesis to provide faster Ultralisk movement speed and Chitinous Plating for improved Ultralisk armor.
The Ultralisk Cavern is built in the late game. In the ZvZ match-up, it is very rare to see such a Hive-tech building being build as most battles end at the Lair stage.
In the the ZvT and ZvP match-ups, the Ultralisk Cavern is built late, with both upgrades researched to optimize the attack and armor of the Ultralisk.