Some photographers ideologically don't believe in using any auto facilities. I find Auto ISO handy in fast changing situations. My Nikon D750 allows me to use auto ISO in Manual mode. This allows me to select shutter speed and Aperture and shoot quickly without worrying about exposure. I try to ensure that I set the upper ISO limit to suit the situation. Also, I can remove minor noise effectively in Lightroom if high ISO results in some noise in low light.

See this article from slr lounge for a favourable perspective:

https://www.slrlounge.com/auto-iso-manual-mode-best-auto-exposure-mode/