In the File Entity module, what is the purpose of multiple "Enabled displays" and "Display precedence order" for a single display mode?
I just can't think of a use-case where I would need more than one "display" for a single "display mode", where the first would not "validate" and therefore look at subsequent ones that are defined.


It's not a feature that I expect it used very much, but that is exactly the purpose.

An example, where one looks at the user agent, could be:

Let's say you're showing a video, a video player is your primary option.

Then there's a user agent that you recognize as a screen reader. Suddenly, showing video doesn't make sense. However, the audio still does, so you fall through the first option, to your second, which is an audio player, that only pulls in the audio stream, significantly reducing the bandwidth usage for both server and client.

Finally, a really low powered mobile device comes along, and then you simply default to displaying an image of the videos first frame, together with a link "Mail me this video", making it convenient for the user to find the video again when using a more powerful computer.