From what I've heard, redundancy and scalability in a single machine (like, you can hot-swap CPUs on a running machine and have lots of redundant RAM, etc.) and being able to run an OS with backwards compatibility all the way to the fscking 1960s or whatever.
That OS is… something: https://medium.com/@bellmar/hello-world-on-z-os-a0ef31c1e87f