You right-click a picture, choose Save image, and get a .webp that your editor, your phone's gallery or a job-application form refuses. JPG fixes that: it is accepted practically everywhere. JPG cannot store transparency and is lossy, so it suits photographs best. For logos, screenshots or anything with a transparent background, PNG is the better target.
JPG has no transparency, so see-through areas of a WebP are filled with a solid colour, white by default; you can change it under Options. JPG holds a single still image, so an animated WebP does not stay animated. The defaults suit most files. Under Options you can set the quality, aim for a target file size and resize the result.
If the WebP is animated, only its first frame is converted and the result is a still picture. Camera metadata such as EXIF and GPS location is not copied into the JPG, and the photo's rotation is applied so it stays upright.