When the AS/400 came out circa 1989 or whatever, you could replace an entire mainframe with a box not much bigger than a mini fridge. The hardware is built for high reliability, and the OS and application software stack have a lot of integration. If Unix is "everything is a file" then AS/400 is "everything is a persistent object in a flat 64 bit address space."

The result is a system that can handle years of operation with no downtime. The platform got very popular with huge retailers for this reason.

Then in later years the platform got the ability to run Linux or Windows VMs, so that they could benefit from the reliability features.

High capacity, super reliable box that you could run your entire business stack on, if you could afford it.