The one I see most is the hero product screenshot. It's a full desktop dashboard captured at 1440px, so at 360px every label in it is too small to read and the image says nothing. A crop of one panel, the part that shows the result, works much better on a phone, and the picture element lets you serve that crop only on small screens. The same goes for hero videos: they need muted and playsinline to autoplay inline on iPhones, and a poster image so there's something to see before the video loads. Without the poster, the top of the page is an empty box on slow 4G.