Why does a cache need an eviction/TTL strategy instead of just caching forever?

Without expiry, cached data drifts out of sync with the real database the moment the underlying row changes, serving stale data indefinitely. A TTL (`@Cacheable` with a configured Redis expiry) bounds how stale data can get, and explicit `@CacheEvict` on update/delete operations invalidates the cache immediately when you know the data changed, rather than waiting out the TTL.