I had the same question recently and although high availability is certainly one reason an equally and perhaps more important reason is "cost per transaction."

I had the same question recently and although high availability is certainly one reason an equally and perhaps more important reason is "cost per transaction." Think of the number of transactions entities such banks and airlines perform a day. This article talks about this in the section "what is a mainframe today": https://www.nanalyze.com/2017/10/ibm-mainframe-computers-today/ https://www.nanalyze.com/2017/10/ibm-mainframe-computers-tod...