Re: I'd like to have an honest non-marketing answer to the question...
I'd like to hear a good answer to why Linux on z rather than on another platform?
Sometimes simply because the organization already had a big investment in z and was testing the waters for migrating some of the application load to Linux. Often the applications in question were written in Java, so moving them was relatively easy.
When zLinux first came out, virtualization was also a big selling point. The z hypervisor (basically a stripped-down version of IBM's VM OS, the granddaddy of hypervisors) could accommodate a huge number of tenants - I recall Usenet posts describing tens of thousands of zLinux LPARs on a sysplex. There are use cases for that sort of thing, and doing it with, say, VMWare on x86 hardware around the turn of the century would have been a hassle, particularly to manage.
These days with containerization (and particularly the rapid growth in management and orchestration tooling) the virtualization features zLlinux are no doubt less of an advantage.
But I still see a fair number of organizations investing in zLinux, and they have some reason for doing so. We've even had customers put our mainframe-environment emulation systems on zLinux, running CICS, IMS, and batch workloads under zLinux rather than in conventional zOS LPARs.