I'd suggest you check out Google's philosophy on Material Design [1] - it explains it quite well.
HN readers are probably not the intended audience for MD. It was basically intended so that layperson users would intuitively understand how to use an interface without thinking too hard about it - e.g. which areas are clickable? Did I really press that button (The ripple and/or bounce effect gives them a visual confirmation that yes, they indeed did). Items appear to float above each other to give the idea that there is more content underneath, or that they can drill down further etc.