Advantages of an f/1.8 lens

This is 'Night and Day', sculptures in marble by Michelangelo in the New Sacristry of the Medici Chapel in Florence, Italy. What is interesting about this photograph is that despite the dim available light, I was able to hand hold the camera using only 800 ISO because I was shooting with the Sigma 14mm f/1.8. Had I taken this with an f/4 lens, say the 24-105mm, the ISO would have been 4000 to get the same exposure. Depth of field wasn't an issue because I stood far enough away from the famous artwork that the entire wall would be sharp at any lens aperture -- thus f/1.8 worked. My shutter speed was 1/160, and I used daylight white balance since the interior was lit by high windows.