There are two kinds of JPGs in this world: baseline and progressive. Progressive is just better. Ann Robson shows us why.
Experimenting locally with a throttled bandwidth, an 80K progressive jpeg beats a 5K baseline jpeg (the same image, downsized) to the page in Firefox on Windows. This should blow your mind. Sure, the progressive jpeg’s first pass is low-resolution, but it contains as much information, or more, as the small image. And if you are zoomed out, perhaps on a mobile device, you will not notice it’s low-res. That’s responsive images working for us right now!
And that's just one reason. Sadly, her research suggests better than 9/10 JPGs on the internet are baseline. Fairly small sample size, but still.