Why you still need JPG
WebP is supported by every current browser, so the reason to convert is almost never the web. It is everything else: photo frames, older printing workflows, some CMS uploaders, camera software, and a long tail of desktop applications that never added a decoder. JPG remains the format almost nothing refuses.
Both formats are lossy, which means this conversion is a re-encode rather than a repackage — the image is decoded to pixels and compressed again by a different algorithm. A small amount of quality goes each time, so convert from the best source you have rather than from something already converted twice.
What does not survive
WebP supports transparency and animation; JPG supports neither. Transparent areas are flattened onto white, and an animated WebP yields only its first frame. If either matters, convert to PNG instead, which keeps the alpha channel intact.