A downloaded WebP may be difficult to use in a specific presentation editor, document workflow or graphics application. PNG gives you a still raster image that can carry the decoded source transparency. Use it when that format solves a real compatibility problem, especially for an icon, diagram or cutout that needs an alpha channel.
Start by checking the original. If the image was saved as a small website preview, the converter only has those pixels to work with. Download the full-resolution asset from its legitimate source if available. Do not enlarge a tiny icon and expect clean new detail. Keep the original WebP until you have confirmed the PNG works in the receiving application. This workflow converts a local file; it does not download restricted assets, remove a watermark or recreate an editable vector drawing from a raster image.