A JPEG white paper titled JPEG XL Image Coding System explains that this new file format “fills the specific needs for responsive web, wide color gamut, and high dynamic range applications” and “provides legacy transition features and effective compression at high visual quality.

Key features of the JPEG XL codec are:

1. improved functionality and efficiency compared to traditional image formats (e.g. JPEG, GIF and PNG),
2. progressive decoding (by resolution and precision),
3. lossless JPEG transcoding,
4. support for both photographic and synthetic imagery,
5. graceful quality degradation across a large range of bitrates,
6. perceptually optimized reference encoder,
7. support for wide gamut and HDR,
8. support for animated content,
9. efficient encoding and decoding without requiring specialized hardware.
10. Lossless JPEG transcoding reduces JPEG size by around 16% to 22%
11. JPEG XL is visually lossless at about half the bitrate required by JPEG.

http://ds.jpeg.org/whitepapers/jpeg-xl-whitepaper.pdf

In short - it makes zero sense at the age of HEVC based formats.