Make the ordinary corrections a photo usually needs, in one place and without installing anything. Aimed at quick fixes before you send or post an image, rather than at replacing a full desktop editing suite.
Adjust light and colour, add a border or a caption, rotate — entirely in your browser.
Each save to JPG re-compresses the image. If you expect several rounds of editing, keep an intermediate copy as PNG and only export to JPG at the very end.