I agree, I was a little confused trying to understand images on the web in the beginning, but after building a while you get the hang of it. HiDPI simply gives you the ability to add an image with - 2x the resolution in pixels, but with the weight load of an image at 1x. Basically, doubling the pixels for cleaner image at higher screen resolution when needed. Just in case the site is viewed on a larger screen than expected when built.
That’s about all you’ll need to know, studying this can be too much if you get carried away. If you want to go deeper, here are a few articles to read (and a WF forum thread):