> Is there a good writeup about why people want this over other, existing formats?
All the existing JPEG files can be converted to JPEG XL while gaining 20% size and still having all the exact same data. There are, what, tens or hundreds of billions of JPEG files out there for which the "original" (RAW or anything) is long gone (or never existed) and people don't want a "photocopy of a photocopy".
You can even decompress the JPEG XL back to the original JPEG file, bit for bit.
For that alone there shouldn't be any question: it's a wonderful feature.
In addition to that Apple / Safari are going to support JPEG XL and there are huge number of applications supporting that format.
People don't want this "over" existing formats. They want this in addition to other formats.
I'm not aware, but the gist is that it's in some circumstances better than AVIF in size and/or quality. Both of the new formats are wayy better than good old JPEG and PNG, but AVIF is the one Google is pushing.