According to Adobe, JPG/JPEG utilises lossy compression to save space. It is basically the reason JPG is used to save camera photograph (because camera usually only have limited storage space, even with external storage card installed), but if the compression level is set to too high (compress too "hard") or a picture is compressed repeatedly the image can be completely ruined in the compression process.
PNG, on the other hand, employs loseless compression, which will save all pixel information intact, even with the highest level of compression selected. But this also made PNG files marginally larger, thus can takes longer to transfer (like, during webpage loading), so it's a compromise between between size and quality.
Another big advantage of PNG (against JPG) is that it supports saving transparent information (in a separate alpha layer), so it's suitable for websites that uses different backgrond color, or has dark mode support. Though that one probably isn't a point of concern for manga files here.
https://www.adobe.com/creativecloud/file-types/image/comparison/jpeg-vs-png.html