Purpose

The reincarnation of the Swarmlord appears to be a stress-induced response by the hive fleets, one triggered when its prey cannot be defeated through biological adaptations alone. Indeed, each time it has been reborn, the Swarmlord has been created with the express purpose of out-thinking the enemy and developing new strategies to achieve the greatest results with the warrior-beasts fighting around it. To this end, the Swarmlord possesses more autonomy than any other Tyranid creature yet witnessed. The Swarmlord combines its own resourcefulness with tactical knowledge and experience accumulated through aeons of bloodshed.

Codex: Tyranids 6th Edition