Every time you upload an image, Simple WebP generates a .webp version. It then alter the HTML to replace img tags with picture tags, which let the browser pick the best format.
WebP images are often smaller than original .jpeg or .png one, so can save bandwidth and load time. Most browsers support WebP nowadays. You can use Google Pagespeed tool to see the gain you would get from WebP.
You will get much faster load time, and better ranking in Google Search.
Simple WebP is very much inspired by the more complete (and complex to install) WebP Express plugin. It also use the same libraries :