The file you saved from a browser, that nothing will open
This is the situation almost every time. You right-clicked an image on a website, saved it, and got a .webp file. Now the application you want to put it in - an older version of a design tool, a document editor, a photo application, an upload form with a fixed list of accepted types - does not recognise it, and offers no explanation more helpful than refusing.
Converting to PNG solves that permanently, because PNG is accepted by essentially everything written in the last twenty-five years. Transparency survives the trip in both directions since both formats have an alpha channel, so a logo saved as WebP stays a logo with a transparent background.
What the conversion does and does not fix
If the WebP was lossy - and most WebP images on the web are - the compression artefacts are already in the pixels. PNG will store them perfectly and losslessly, which means the converted file is larger and looks exactly the same. As with every lossy-to-lossless conversion, nothing is recovered; the file simply stops degrading further.
That is fine, because recovery is not why you are here. The goal is a file the next piece of software will accept, and the conversion achieves that exactly. Just do not be surprised when a 200 kilobyte WebP becomes a PNG several times the size.
If it is going back on the web, do not do this
Converting WebP to PNG for use on a website undoes the reason WebP exists. The same image will be substantially heavier, which costs your visitors on mobile connections and costs you on every rendering metric that depends on how fast the largest image arrives.
Modern browsers all support WebP, and the fallback pattern for the handful that might not is a picture element with both sources listed. Convert for compatibility with desktop software; keep WebP for anything a browser will display.