File size
At matched visual quality WebP lands roughly 25-35% below JPG. On a photo-heavy page that is the single largest performance saving available, and it feeds directly into Largest Contentful Paint.
Quality at the same size
Given equal file sizes, WebP generally holds detail better and shows less blocking in flat areas like skies. JPG's DCT compression is a 1992 design; VP8 had eighteen more years of research behind it.
Transparency and animation
WebP carries a full 8-bit alpha channel in both lossy and lossless modes, and supports animation. JPG does neither — no transparency at all, no animation. This alone rules JPG out for logos and UI assets.
Compatibility
This is JPG's entire case, and it is a strong one. Every device, every editor, every printer, every email client and every upload form ever built accepts a JPG. WebP works flawlessly in browsers and unevenly everywhere else.
Which to use
Serve WebP on your website. Hand out JPG when the file is going to a person, a printer, an email or a form. In practice most teams do both: WebP in a element with a JPG fallback beneath it.