What problem do Cache Components and the `use cache` directive solve?

Before adoption I inventory route dynamism, personalization, mutation invalidation, deployment adapter support, and multi-instance consistency. A cached component is treated like a shared materialized result: its inputs, lifetime, tenant boundary, and purge path are explicit. Static export does not support the same runtime model, and streaming must work through the hosting chain for the intended performance benefit. I roll out representative routes first and verify HTML, RSC payloads, cache hits, stale behavior, and cross-user isolation rather than enabling the flag as a cosmetic optimization.