Better compression (tldr faster load time) and less quality loss. That, and WebPs can be animated.
PNGs are more for infographics, insane detail images, yadayada.
I'm talking in the context of on a general webpage; not about it being supported on a site or not (that's a UX perspective by technicality, allowing users to upload it or not as media).
That's the reason it exists; better compression at the coss of less quality loss while being animatable.
I actually know how to get around this without extensions. When you go to save the image, go to the bar that says WEBP, scroll down to All Files, and add .png to the end of the file name.