When WebP is the right export
Use it for photos on a page you control. Skip it when the host only accepts JPG, which is common on older forms and some printers.
Quality is yours
You pick the number. Quality 98 or higher uses lossless WebP, which suits flat UI better than a mushy 60. Photos usually want something in the 70s.
Transparency
PNG and WebP can keep a transparent background. JPG cannot. If you just cut a white backdrop, export PNG or WebP, not JPG.
Size after the conversion
This page changes the format. If you also need a hard cap such as 200 KB, open Compress to KB with the file you just saved.