Comment by PaoloBarbolini
3 months ago
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.
At they very least it will take time. Rotating is a fairly common operation, even simple photo viewers often have buttons to quickly rotate the image. Being able to do this efficently is beneficial.
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.