i'm still a noob and can be completely wrong, but climbing is generally pretty logical so i'll tell you what i think.

nuts are racked together because, you can fit a bunch of them on 1 biner, the rest of them are a good handle to set the placed piece, and nuts have a smaller range than cams, so you might want to have a multitude of sizes readily available in case you chose the wrong size initially. a nut that's not tightly set could pop out if clip directly to it, that's why extended draws are reguarly used.

i think people rack a cam per biner because they take up more space, it would be hard to clip more than 2 small or 3 large cams on 1 biner and have them hang nicely. also you have more range with them so you're less likely to chose the wrong piece. i actually rack 2-3 cams per biner because its less gear to carry up and i like having the next size up or down - like i said, i'm a noob.