Proxmox simplifies storage, media, resource sharing, ZFS, Clustering, and Management.

It supports modern storage protocols so offloading your storage to a NAS is super easy.

I would say the biggest advantages are software package version control, stability, platform support, and compatibility. It makes VM management super simple without needing to be an expert in getting the entire KVM stack working properly.

Couldn't you roll your own peertube and stream videos using their video protocols rather than transcoding?