Okay I'll explain it one last time.
In the streets having a crystal clear frozen moment with a good depth of field is important. That calls for:
1) a fast shutter speed, as fast as possible, something like 1/1000
2) an aperture between f5.6-f11.
I've already asked you to do an exercise and put your camera in manual mode. Dial in those settings, the image will be underexposed. To get a good exposure the camera will raise the ISO due to the dark image.
Now if I'm on the street with my camera locked at ISO 800 or 1600 my sensor has more boosted sensitivity to light and is overexposed and so to darken the image the camera will automatically choose a very fast shutter speed and an aperture like f5.6-f11.
So you see, by manually sticking with ISO 800-1600, the camera is giving me the shutter speed and aperture ranges I want automatically as it has a wider range to darken the image. That's good for the streets as I will always have a fast shutter speed and a great depth of field. If my camera has an auto ISO range of 100-800 on, you will NOT get that effect or be able to rely on the camera achieving it as predictably.
So manually staying on P at a high ISO gives me the benefits of a well exposed fast shutter and deep aperture image that I want.>>3310092
Because in nature you want a low ISO to see the animal with no grain and you can sacrifice some aperture (say drop it to f2.8) for the shallow depth of field behind the animal. On the streets you want EVERYTHING in focus so you want f5.6 to f11.