While they had they own datacenters (two of them, hosting about ~2k physical x86 server machines and many more virtual machines) all of their retail shop inventory and sales processing ran on IBM SystemZ mainframes. They had more than one of them, and were in the process of buying a new one.
What they told me is basically they have a 24/7 operation with no downtime accepted. They also need to process a huge number of orders (check-out at the grocery stores, basically) in real time. Their inventory systems were not very different from the original ones written in the late 70ies except for some evolutions along the way.
They also told me about that time when they switched from an old mainframe to a new one, during the business day, in the middle of the operations, with no downtime or service interruption.
So basically what I got from that is that the benefits are:
- write once, run forever code
- 100% uptime (not "five nines", literally 100% uptime)
- a reference environment that you can rely on (no runtime change, everything that works today will keep workin in 30 years)
- value added consulting from IBM
- a complete set of tools (runtime, database, primitives, libraries etc)