Think: opposite of general-purpose operating systems use-cases. Very IO/compute/latency-intensive things. Where the whole programming paradigms are designed around efficient (even real-time) compute. Where the software+hardware pair are written once and rarely if ever changed, restarted, or taken down for maintenance. Credit card transaction processing and airline reservation systems for example.

Of course you could use Linux or similar for such jobs, but highly specialized hardware+software pairings can eke out surprising performance at the cost of massive hardware and software-engineering bills.