The envisioned use case is Kata Containers https://katacontainers.io (formerly "Clear Containers"), which behaves like a container runtime but is actually a virtual machine with its own kernel to enhance isolation from the host. Since it runs its own kernel, it needs something like this to access files on the host.

The advantage compared to running normal VMs is that it's interoperable with things that expect a container runtime like Docker or rkt - e.g., Kubernetes can run Kata Containers - and the overhead / density / startup speed is much closer to that of actual containers (in the sense of namespaces+cgroups) than traditional VMs.