Why is ISO still relevant?

Something I'm trying to wrap my head around... why is ISO still important in digital photography? I wonder, can someone please provide a technical answer as to why aperture and shutter speed are not sufficient? I "got it" when we were talking about film. Today's cams do a pretty darn good job of calculating all three points of the exposure triangle all on their own. Is this a legacy feature or something just to support that "grainy" look that you may want now and then? Or are there some physical or mathematical restrictions that necessitate a third variable? Are aperture ranges incapable of capturing enough light in low-light situtations or are modern lenses too full of glass to be fast enough to capture just enough blur in those helicopter rotors?

Well, now that I'm thinking it through, it's getting a little more clear but I'd appreciate a scientific explanation. All I seem to find on the 'net is "this is the way it is and this is how to use it" but no real reasoning behind it.

Thanks in advance for your help and understanding.