Are we looking at the same graphs? In the linked comparison, WebP has at best maybe 10% of an improvement over JPEG for a 500px image, and actually performs worse as the images get bigger. (The 18% you’ve likely read are in comparison to the cjpeg, not mozjpeg, which compresses better.)
There’s also no reason one should tolerate further loss just because the image is already lossy. Whether the original is lossy or not, you either want to keep it exactly the same, or you're okay losing quality to save some space. Archiving and preservation come to mind as the first reason someone would want to go with the former.
There’s also no reason one should tolerate further loss just because the image is already lossy. Whether the original is lossy or not, you either want to keep it exactly the same, or you're okay losing quality to save some space. Archiving and preservation come to mind as the first reason someone would want to go with the former.