Yesterday I was visiting Cathedral of Our Lady of the Angels and was trying to take a photo inside. As happens most of the time, the inside was not very bright. Outside was bright and sunny so my camera was set at ISO 100 when I was shooting Civic Center, Dorothy and Disney Halls.
I let my ISO loose LOL. I set up ISO range to 100-6400 and let camera choose whatever it found proper. I didn't change anything else. I was in Av mode with f/8. Now camera chose ISO 6400, maximum that I allowed it to use, and this brought the shutter speed to 1/15. 60 times faster than it needed at ISO100!
See the great shift in the photo sharpness. The photo came out very nice. Also, compared to other cameras that I have used, Pentax K-01 does take nice photos in low light and with ISO as high as 6400, you would barely notice noise in the photos.
In short, in low light, ISO can help you take better photos which are almost impossible with hand holding of the camera at a fixed or low ISO.
Here is one more example of photos with ISO 100 vs ISO 3200.
Photo with ISO 100.
Personal tip: For outdoor photos, I keep ISO to 100 and when I get in low light or need to shoot a fast moving object, I increase ISO to as high as 6400 on Pentax K-01 but on my Nikon S9300 or Panasonic FZ28, I would hesitate to use more than ISO 800.