"Despite myriad studies, there is still no consensus on why sleep is needed for survival."

https://www.nature.com/articles/d41586-025-00964-w (2025)

Probably because it evolved very early (like before bilateral symmetry, multi layer body cavity, or kidneys early... maybe even before multicellular animals early) and so has been incorporated as an essential pillar into multiple processes layered on top of that fundamental architecture.