And if you already have compression artifacts, what use is lossless?

To further reduce file size without further reducing quality.

There are probably billions of jpeg files out there in the world already encoded in lossy JPEG, with no corresponding higher quality version actually available (e.g., the camera that captures the image and immediately saves it as JPEG). We shouldn't simply accept that those file sizes are going to forever be stuck, and can think through codecs that further compress the file size losslessly from there.