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.

And the advantage compared to running containers is...?

> a virtual machine with its own kernel to enhance isolation from the host

Whether that’s something you care about or think is necessary is left as an exercise to the reader (i.e. it depends).

Too short text for that topic. Container world certainly needs a lot of storage problems resolved. So, why do containers not need this? What part of it do containers not need?

Containers have no access to virtio, so they can't use this. It's that simple.

Containers are not virtual machines.