A handy mostly straightforward UI with built in backup/restore and other useful tools.

It's hardly a requirement but if someone is just starting to learn, proxmox has lots of documentation on how to do things and the UI keeps you from footgunning yourself copy/pasting config code off websites/LLM too much.

Personally, I didn't want to manage my management/virtualization layer. I wanted something that was an all-in iso that wouldn't tempt me to configure at all. I wanted to be able to restore just my container backups to a new PM install without worrying about anything missing at the host (to the extent possible).

I also like that Proxmox can be fully managed from the web UI. I'm sure most of this is possible with LCD on some distro, but Proxmox was the standard at the time I set it up (LXD wasn't as polished then)