Headless devices are everywhere now.
Mac minis, Raspberry Pis, Linux systems, mini PCs, cameras, and embedded devices often run perfectly fine without a dedicated monitor attached - until you suddenly need direct access to the screen.
That is especially true during setup, debugging, recovery, or when remote access is not available yet.
Using an iMac as a monitor is surprisingly straightforward. While modern iMacs no longer support direct HDMI input, inexpensive USB HDMI capture adapters provide a simple workaround.
Most capture adapters appear on macOS as standard USB video devices. Combined with viewing software, the iMac can display the incoming HDMI signal in real time, including audio.
For this, I use:
AV Monitor Pro
The app supports USB video devices, HDMI capture adapters, fullscreen preview, audio monitoring, recording while staying lightweight and easy to use.
For headless systems especially, it turns an existing iMac into a flexible external display and AV monitoring station.