My best guess would be to have a single point of management for both LXCs and VMs.

LXD/Incus provides a management and automation layer that really makes things work smoothly. With Incus you can create clusters, download, manage and create OS images, run backups and restores, bootstrap things with cloud-init, move containers and VMs between servers (even live sometimes) and those are just a few things you can do with it and not with pure KVM/libvirt.

Another big advantage is the fact that it provides a unified experience to deal with both containers and VMs, no need to learn two different tools / APIs as the same commands and options will be used to manage both. Even profiles defining storage, network resources and other policies can be shared and applied across both containers and VMs.

Incus isn't about replacing existing virtualization techniques such as QEMU, KVM and libvirt, it is about augmenting them so they become easier to manage at scale and overall more efficient. It plays on the land of, let's say, Proxmox and I can guarantee you that most people running it today will eventually move to Incus and never look back. It woks way better, true open-source, no bugs, no BS licenses and way less overhead.