It's useful on a headless Mac used a server.
Tried doing Steam link to a Windows computer without monitor attached and would only work in like 1024x768, needed a monitor attached to get 1920x1080 almost bought a dummy for the Windows machine similar to this but had and extra monitor so I didn't bother.
Also VNC like system will turn on a monitor and you can see what people are doing remotely, RDP doesn't do that. RDP is a really nice system that gets really good performance over low bandwidth links due to remoting higher level graphics API, at least it used to, wish Apple had a built in equivalent that worked similarly.