There are a lot of systems that rely on precise timing, for instance clock generation/recovery, or opening/changing/closing a control valve in some process. It

There are a lot of systems that rely on precise timing, for instance clock generation/recovery, or opening/changing/closing a control valve in some process. It has to be precise or you lose sync in the first case or legitimate catastrophes can happen in the second. A realtime OS makes some guarantees about the timeliness of things like interrupt service routines, and that necessarily excludes unbounded and unknown workloads from getting in the way -- something that every general purpose or soft realtime OS struggles with as the lack of determinism can improve throughput and scalability.