The reasons why I do grain into strike water. (1) I can pour the grains in as I stir to make sure I don't get dough balls, (2) I don't have to worry about grains on the bottom not wetting out and sticking to the bottom of the kettle or mash tun and lowering efficiency, and (3) it is easier to life a bucket of grains and pour than a full kettle of very hot water and pour (though I suppose if you have an HLT and gravity or pump the hot water in, that point would become moot).
Reasons that I can see why people may want to pour the water into the grains: (1) grain temperature comes up slowly (instead of the first grains being overheated and the water volume slowly cooling down to mash temperature), and I am sure there are other reasons but it really doesn't seem apparent to me right now.