WebP is an image format designed for efficient web delivery. It can store photographic images and can also support transparency, which makes it useful for websites that want smaller image transfers.
Why you see WebP files
A website may deliver WebP because the browser supports it and the file can be smaller than an equivalent older format. Saving an image from a page can therefore produce a .webp file even when you expected JPG.
When JPG is useful
If another app, form or older workflow does not accept WebP, converting a photographic image to JPG can improve compatibility. Remember that conversion does not restore details that were already removed by earlier compression.
When PNG is useful
PNG is often a better choice when you need transparency or lossless flat graphics. For photographs it may produce a much larger file.