The key benefit is confidence: if staging and production use the same image, you know production will behave like staging. The key challenge is handling anything stateful. Separate state from compute.
Strong answers explain: never modifying running infrastructure (replace instead of patch), benefits (reproducibility, consistency, simplified rollback), challenges (longer deploy cycles, handling state, cultural shift from SSH-and-fix), and prerequisites (automated image building, good CI/CD, external state management). Best candidates discuss when immutable infrastructure is appropriate and when it is not.
Tests modern infrastructure philosophy. Candidates who still SSH into production to make changes are operating in an outdated model. Those who understand immutable infrastructure are thinking about reliability at scale.