The point is that you want management, easy ways to create images, backups, move container between hosts, orchestration, network management and sometimes not only container but also virtual machines. LXD does it all very well and if you don’t want those resources you might as well use systemd-nspawn.
They’ve taken over Proxmox. Not sure if you’re following but they have now a WebUI and the entire solution is magnitudes better than the crap Proxmox has been offering.