The Lair is a key building for Zerg technology. The Lair is the second stage of the Hatchery tech tree. Originating from morphing of the Hatchery, the Lair can then be morphed into the Hive after the Queen's Nest is built.
The Lair allows Zerg to build a Spire as well as research Lurker ability at the Hydralisk Den. Many Zerg strategies involve quickly upgrading to Lair tech to get access to these units, leading to Lairs sometimes being morphed in the natural, rather than the main base, to escape Comsat Scans. Obtaining a Lair also unlocks several key Overlord upgrades, increasing their usefulness and likelihood for survival on the battlefield.