I find Auto ISO is most useful when I have a desired shutter speed and aperture but the light level varies. An example of that is a football game where a player moves from full sun into a shadow. There is no time to adjust ISO but Auto ISO makes the adjustment. Aperture Priority would address the scene with a slower shutter speed - maybe too slow for a sharp image. Shutter Priority would adjust aperture - but if you were already wide open it would simply create a dark image. Manual exposure with Auto ISO would give you a correct exposure at the lowest possible ISO without changing your shutter speed or aperture.
Auto ISO is also useful with high ISO levels to drop the ISO if you have adequate light.
I don't use auto ISO if the light on the subject is constant with changing backgrounds. And I don't use ISO routinely - in most cases I have time to select the correct ISO and trade off as needed.