Ansel's use of heavy filters and burning to get black Sierra skies. Yes, they are not 'real' skies, but for those of us who have spent afternoons under those skies with thunder storms rolling in like clockwork, shaking the air around you at 11,000 feet, those photographic black skies and white clouds
feel like or represent the reality of being there -- a threatening sky. One is not suppose to be comfortable seeing anything close to a black sky.
Careful manipulation can help to induce an emotional response from the viewer due to that 'enhanced' reality. Actually, every decision made at the enlarger is changing the original representation of reality (the negative). Burn the corners in to keep the viewers' eyes within the image and one has altered the reality of the altered reality of the negative. So to speak.