I can kind of see it if you want to make sure you have the right cereal to milk ratio. Not that I do it that way, I just eat my cereal dry the times I forget to pick up more milk.
The way I see it, the cereal is the milkshake and the milk is the oreo pieces. I wanna have a decent ratio, but better believe I'm guaranteed a heapin helpin of cereal. Plus, if there's not enough milk, you can just toss it in the milk like a salad. Win win win.
I pour milk into one of those wide coffee mugs and then take a spoon of cereal directly from the box/bag, lower it slowly into the milk just enough to get milk into the spoon and then eat it.
The only time pouring cereal into milk is viable is when you have extra milk after finishing your cereal and are still hungry. I refuse to believe in any other way!
Also I've always done cereal first but I think that's partially because I have a variety of different cereals and don't always want the same amount of milk.
Pouring the milk over hydrates the cereal more evenly. Especially corn flakes are prone to float on the milk's surface if you add it into a bowl of milk.