File explorer support, thumbnails, local image viewers and editors, import into other media software like slideshows, video editors, 3d modeling software, etc etc.

macOS and Windows’ respective file explorers and built-in image viewers both support it, as do Photoshop, GIMP, Krita and a few more. https://en.wikipedia.org/wiki/JPEG_XL#Official_software_supp...

The encoder currently isn't using a lot of format features like curves or layers (which would partially also require deeper integration in for example Photoshop to pass such information to the encoder).

I suspect there are a few 10s of thousands of websites that still only accept Jpeg (as a user upload). Some accept PNG. Few accept HEIF, almost none accept JPEG-XL

This is probably going to be the longest tail of them all.

I only accept webp, jpg, and png atm in my SaaS product. I should probably add support for HEIF right now at least. All iPhones are using that, so it's hard to get around it.

Aren't they translating to JPG on upload automatically anyways?

That must be happening, because I haven't gotten any complaints and I know we have many users on ios.