I found this video to visualise what tone mapping is trying to achieve, and why "photorealism" is hard to achieve in computer graphics: https://www.youtube.com/watch?v=m9AT7H4GGrA

And I indirectly taught me how to use the exposure feature in my iPhone camera (when you tap a point in the picture). It's so that you choose the "middle gray" point of the picture for the tone mapping process, using your eyes which have a much greater dynamic range than a CCD sensor. TIL.