WebP is now the default format for images saved from Chrome and many websites, but plenty of tools, older editors, and upload forms still only accept JPG. This decodes the WebP and re-encodes it as JPG entirely on your device, so you get a file that opens anywhere without waiting on (or trusting) an upload-based converter. Since WebP supports transparency and JPG doesn't, any transparent areas are flattened onto a white background during conversion.
The format you did not choose to download
Most people meet WebP by accident. You save an image from a website, and instead of the JPEG you expected you get a .webp, because the site was serving the smaller modern format. Then something refuses to open it: an older photo editor, a print shop's upload form, a piece of desktop software that has not been updated in a while, or a device that predates the format.
Converting to JPEG is the pragmatic answer. You give up some efficiency in exchange for a file that has been universally readable for three decades.
Two things to expect in the result
If the WebP had a transparent background, that transparency cannot survive, because JPEG has no alpha channel. Those areas are filled with white. For a photograph this is irrelevant, since nothing was transparent to begin with; for a logo or a sticker it is the difference between something you can place anywhere and something that arrives on a white rectangle.
The other is that the source was almost certainly lossy already, so the conversion adds a second round of lossy compression on top of the first. At a sensible quality setting this is not visible, but it is a reason to convert once from the original rather than repeatedly through a chain of formats. If you have access to whatever the site generated its WebP from, that source is always the better thing to convert.