• Existing JPEG files (which are the vast, vast majority of images currently on the web and in people's own libraries/catalogs) can be losslessly compressed even further with zero loss of quality. This alone means that there's benefits to adoption, if nothing else for archival and serving old stuff.
• JPEG XL encoding and decoding is much, much faster than pretty much any other format.
• The format works for both lossy and lossless compression, depending on the use case and need. Photographs can be encoded in a lossy way much more efficiently than JPEG and things like screenshots can be losslessly encoded more efficiently than PNG.
• The format anticipates being useful for both screen and prints. Webp, HEIF, and AVIF are all optimized for screen resolutions, and fail at truly high resolution uses appropriate for prints. The JPEG XL format isn't ready to replace camera RAW files, but there's room in the spec to accommodate that use case, too.

It's great and should be adopted everywhere, to replace every raster format from JPEG photographs to animated GIFs (or the more modern live photos format with full color depth in moving pictures) to PNGs to scanned TIFFs with zero compression/loss.