The image could have been encoded with a high compression ratio, or even something like OxiPNG. In that case, while re-encoding it wouldn't lose quality, it could still have the side-effect of making the file bigger.
You can also have other situations where this is useful like a primarily hardware pipeline that doesn't support rotation, but you can mark the rotation at the end. Although this is probably less of an issue for PNG than formats that typically come out of cameras and scanners.