A fast-moving Scarlet Honeyeater flapping its wings in full sunlight needed a high ISO setting.
The tiny Scarlet Honeyeater in the photo above was constantly moving. I needed a fast shutter speed (2,500th second) to freeze its movement. I also needed a fairly small aperture (f/10) to get enough depth of field. Combine those two things and, even in bright sunlight, I was going to end up with an underexposed shot. But winding the ISO up to 1,600 made that problem go away. It gave my camera the “superpowers” it needed to get a sharp image of a fast-moving small bird.
So, don’t think of high ISO as being only necessary in weak light. Think of it as being useful any time that the light is simply not strong enough for your other settings.