What convert JPG to WebP actually buys you
Bandwidth, and not much else. WebP uses a more modern codec than JPEG, so at a matched visual quality it needs roughly a quarter to a third fewer bytes. On a page with twenty photographs that is the difference between a fast site and a slow one, especially on mobile connections.
Nothing else about the image changes. Both formats are lossy, both are rectangular, and a JPEG has no transparency for WebP to preserve. This conversion is purely a size optimisation, which makes it the easiest one to reason about on this site.
The double-compression question
Worth understanding before you convert a whole library.
Your JPEG is already the result of throwing detail away. Re-encoding it as WebP throws away more, and the second pass cannot know which artifacts came from the first. The usual result is that WebP faithfully preserves the JPEG's own block structure while adding a little of its own.
At 90 percent quality and above this is genuinely hard to see on photographs, so for a bulk conversion of web assets it is a reasonable trade. If you still have camera originals or unedited exports, converting those instead skips a whole generation of loss and gives a visibly cleaner result at the same file size.
Choosing the quality
Match the source if you know it. A JPEG saved at 85 gains nothing from being re-encoded at 100: you get a larger file that preserves the existing artifacts in more detail.
If you have no idea, 90 to 92 percent is the sensible default. It keeps most of the size advantage and stays visually indistinguishable on photographic content, which is what almost all JPEGs contain.
JPG and JPEG are one format
Worth saying because it splits people across two searches. The three-letter extension is a holdover from filenames that allowed only three characters after the dot, so a search for jpeg to webp and a search for jpg to webp want the same result. This page accepts both extensions without distinction.
What to do about older clients
Browsers are not the problem any more. The gaps are in email clients, some desktop applications and a scattering of older Android system webviews. For a website, serve WebP directly. For an email attachment or a file someone will open in unknown software, JPEG is still the safer thing to send.
Runs entirely in your browser
Both the JPEG decoder and the WebP encoder are already in your browser. Nothing is uploaded, so a folder of several hundred photos is limited only by your own patience.