Why this conversion is usually necessary
Images saved from the web are increasingly WebP, and a great deal of software has not caught up. Older versions of Office and Photoshop, many photo printing services, and a large number of upload forms accept JPEG and PNG only.
Downloading an image only to find nothing will open it is a common and frustrating discovery. Converting to JPEG solves it, at the cost of a small further quality loss and a larger file.