Downloadable image - what is it?

[Miku]

Hi,
I see an option 'Downloadable' in product images' properties. Could someone explain what the feature can be use for? I mean I have been searching for an option to place an image (high resolution - with the result that the file is a larger size) and make it available only for download. So the image hasn't loading itself while specific product is loading but is only visible as an option to download - for customers who want to see a better quality image. Is that possible with that option? I have ticked the option but nothing happens -  the image is visible in the product details in the same way as the remaining ones set as 'Displayable'.

[Miku]

On Vm wiki I can see:
"Choose "displayable" to allow customers to view the media only, or "downloadable" if you want them to be able to download the media" I don't really understand.... What does that mean "if you want them to be able to download the media"? How does it work in practice? Can anyone clarify me? Maybe the feature refers only to media other than image file, mp3 file for example?