SS319 wrote:
Because the picture was already in the JPEG format when he chose to level it. Any adjustment that moves or combines pixels in a JPEG will severely reduce resolution. If JPEG is your thing, then more of your photography must be done in front and behind the camera, while if you record the RAW image, you can adjust the image without reducing the resolution nearly as much and thus you can just record the incident light upon the sensor in your camera.
Or even to shoot both.