The argument for it is only lossless and progressive rendering. So if you have large files, it might be better to serve it as JXL, so it can show the image at least in some quality before it fully loads (for slow connections). If you have lossless files, then it's better also.

AVIF is better otherwise, so we will probably see AVIF become the new web image standard and JXL have a niche for higher quality images.