Bhyve (FreeBSD's native hypervisor) already contains a 9P over VirtIO "server". The protocols allows virtual machines and their host to share a directory instead of block storage without having to configure and use a network filesystem like NFS. It's even bootable as in the UEFI BootROM should be able to read the bootloader via 9P, the bootloader can use EFI boot services to read the kernel (configuration, kernel image, kernel modules), and the kernel can than mount it as root filesystem. This means that use a directory instead of an image to boot a virtual machine. I can imagine a lot of use-cases for this e.g. a CI/CD pipeline that tests an experimental kernel (feature) inside a hypervisor.

9p is a simple network protocol for implementing filesystems. It originates from the plan9 operating system but its main usefulness nowadays is that it permits people from implementing filesystems in user space without having to integrate deeply into kernels specific interfaces like FUSE.

I.e. if your OS can mount 9p volumes you can implement a user filesystem easily by just running some process on localhost and mount it. And it also works remotely (although that's less useful IMO, although not entirely useless)

It's a virtio filesystem, so it sends 9p.2000L (probably) down a virtio "pipe" so your virtual machine can access its host filesystem