When rotation helps
Photos taken with the camera turned sideways often display upright on the phone but sideways everywhere else, because the orientation lives in an EXIF tag that not every program honours. Rotating the pixels themselves fixes the image for good.
How the result is written
The image is redrawn on a canvas with the transformation applied and saved as a new file, so the change is baked into the pixels rather than stored as a tag. Rotating by 90 or 270 degrees swaps the width and height; flipping keeps the dimensions the same. Nothing is re-sampled, so no sharpness is lost beyond the re-encoding of the output format itself.
Batch handling
The same transformation is applied to every file you add, which makes it quick to correct a whole set of photos from one shoot. Results can be downloaded together as a ZIP archive.