Not just that. Most operating systems lie about when an IO transaction completes for performance reasons. So if you lose power or the IO device dies you still think it succeeded. A mainframe doesn't do that... it also multiplexes the IO so it happens more than once so if one adapter fails it keeps going. The resiliency is the main use case in many situations. That said IME 99.995% of use cases don't need a mainframe. They just don't need to be that reliable if they can fail gracefully.