Different jobs want different image formats. Websites prefer WebP for speed, printers ask for JPG, and logos with transparent backgrounds need PNG. Converting by hand usually means opening an editor you may not have. This tool changes an image from one format to another in the browser, keeping the picture intact.
WebP is usually the best choice because it is much smaller than JPG at similar quality and is supported by all current browsers.
Converting between formats is close to lossless at normal settings, though moving into JPG always involves some compression.
Transparency is kept in formats that support it, such as PNG and WebP. JPG has no transparency, so it becomes a solid background.
Yes, run them one after another, or use the bulk compressor when you have a large batch.