here is FUIF (one of JXL parents) author writing about benefits of progressive: https://github.com/cloudinary/fuif
In short, unlike progressive JPEG, you can serve truncated image files to get lower resolutions
There was also an update to it called FUIF - Free Universal Image Format
https://github.com/cloudinary/fuif
But it too got superseded by JPEG XL (.jxl) https://jpegxl.info/
yeah. it's partially based on https://github.com/cloudinary/fuif so if you use that part of JPEG XL you'll get something similar out of it.