The benefit to me is that I can set whatever combination of the 3 I want to automatic. In reality i tend to just set the ISO to automatic and continue to control my Shutter and apature.
I don't think I can set auto ISO so easily in any other mode - as you have to have it set as auto in the menus - i can't use a dial to go down 100,50,auto.
By allowing me to flex like and also set auto aperture (as auto at the same time) so only leaving me to control the shutter.
I can concentrate on my process, shoot, review, rate between cars coming at 1 minute intervals without worrying about changing light or over/under exposing if I am shooting for freezing the car or a high speed pan.
It just works really really well, without having to change my dial between AV,TV,M etc when working with limited time windows.
The aperture thing works really well as it seems to balance the ISO/Aperture decently with priority on setting wide open if you are risk of under exposure (and not flexing ISO up) but conversely if you have excess light it will stop down a bit and balance the ISO.
Maybe i've just been missing something with Auto ISO for years, but on my 7D it never seemed to work like that, so to come from fully manual shooting or flicking between AV/TV to this kind of virtual assistant has been a bit of a revelation, and I can use thumb control to effectivly flick between TV/AV without overwriting any other setting, unlike when switching modes.