How quickly can you reboot a bank if it crashes? And how do you access your accounts in the meantime?
What do you do if the air traffic controllers' computers suddenly become non-responsive - can you put the airplanes in a suspended state?
Why is it that we take for granted that the largest, most critical organizations on earth will keep functioning, 7x24, 365 days per year?
Because Continuity is a business mandate that is built into the computers that can be trusted to keep the world economy, and other critical areas, functioning.
And many of the software solutions that keep mainframes running so well have this reason for their existence: to keep the business running even if something bad happens to the mainframe. That can include backup and recovery, real-time fail-over to another mainframe (likely elsewhere in the world in case of natural disaster), and just the ability to see a problem coming and get everything in place to prevent or quickly deal with it.
This value of the "why" dimension is worth the entire existence of some organizations. If a bank suddenly stopped operating for hours, it would take a potentially lethal financial hit. If it stopped for days or weeks, it would likely go out of business. So its mainframe computers must have the necessary solutions for continuity to ensure that the bank doesn't crash. And yet, so few non-mainframe environments have ever done a successful recovery test of their entire production computing configuration - or even the most critical part - often, even in organizations where they have a mainframe that has done such disaster recovery testing and planning.
That's one of the reasons why the mainframe is such an important part of keeping the world economy running, and why Continuity is such an important value of the mainframe.