Bhyve doesn't feature nested virt though.
In practice though, most setups don’t actually need it if you’re running workloads directly on the host.
Also, if your goal is testing or simulating clusters, you can already run Sylve inside jails. That gives you multiple isolated “nodes” on a single machine without needing nested virt. We have a guide for it here: https://sylve.io/guides/advanced-topics/jailing-sylve/
So you can still experiment with things like clustering, networking, failure scenarios, etc., just using jails instead of spinning up hypervisors inside VMs.
Nested virt is still useful for specific cases like testing other hypervisors or running Firecracker inside VMs, but for most Sylve-style setups it hasn’t really been a blocker.
Outside of learning and testing I am not sure of what uses there might be but I'm curious to know if there are.
In prod, let's say you run workloads in Firecracker VMs. You have plenty of headroom on your existing hardware. Nested virtualization would allow you to set up Firecracker hosts on your existing hardware.
However, the point about firecracker VMs in place of containers I think is really a good use-case. Firecracker can provide a better isolation environment, so it would be great to be able to run Firecracker VMs for workloads, which would require that the host (and the VM host above) support nested virtualization.