It may be very hacky or outdated, but was new to me. Kind of the CSS3 equiv of using gifs
You use it to prop open a background image or video - essentially to create a proportional space in the pageflow.
If you want content to work without additional pain of the padding set the div to relative and create a child div with any content to fit that space in an absolute aligned to the relative parent. Float nested with a vertical auto center, etc.
I agree it is sounding a bit crazy and painful without an example. Can't do that at the moment. Will try to soon.