dual pulleys are usually due the A/C compressor, which uses dual belt for better traction at the compressor.

can't tell if there is no a diameter like that on single belt, but other than that I can se any other reason. I know there are 2 or 3 ( maybe 4 ? ) diameters of pulleys available and most of them are available on dual and single groove config. I'm sure about two of them are at least.

I guess a large pulley diameter is to keep safe the alt bearings and lowered temp inside on "racing" High Performance cars, prone to be revved up.