When you need to go back from WebP to PNG
WebP is great for the web, but plenty of everyday tools still don't read it: older image editors, PowerPoint or Word templates, some CMS upload fields, and printers all tend to reject WebP while accepting PNG without any trouble.
It's also useful for layer-based editing — some apps with limited or no WebP support handle PNG natively, so converting first avoids compatibility errors when you open the file.