This tool reads your JPG image directly in the browser and encodes a PNG copy, so there is no upload, no queue and no waiting for a server. It works on desktop and mobile.
PNG is useful when you will edit an image repeatedly, because re-saving a PNG never loses further quality. Converting JPG to PNG stops further compression damage, and gives you a format that supports transparency in your editor.
Converting a JPG to PNG does not restore detail already lost to JPG compression, and the PNG will be larger than the original.