1. If the thing that's going to be loaded isn't a JPEG, but rather a PNG, or WebP, or SVG, or MP4...
2. These are usually delivered embedded in the HTML response, and so can be rendered all at once on first reflow. Meanwhile, if you have a webpage that has an image gallery with 100 images, even if they're all progressive JPEGs, your browser isn't going to start concurrently downloading them all at once. Only a few of them will start rendering, with the rest showing the empty placeholder box until the first N are done and enough connection slots are freed up to get to the later ones.
2. These are usually delivered embedded in the HTML response, and so can be rendered all at once on first reflow. Meanwhile, if you have a webpage that has an image gallery with 100 images, even if they're all progressive JPEGs, your browser isn't going to start concurrently downloading them all at once. Only a few of them will start rendering, with the rest showing the empty placeholder box until the first N are done and enough connection slots are freed up to get to the later ones.