the main reason I think is all about weight. Basically before a plane takes off, the dispatcher and the captain must agree on the weight of the aircraft. this is very important as it means they have to have enough fuel to reach the destination plus extra in case of diversions. Heavier the aircraft = more fuel.
I know it seems silly when you have a child and a man and it may seem like its not a big deal, but imagine if everyone did this then the difference could be quite substantial. Airlines estimate the weight. I think they say its average 80 kg for a male maybe 70 for a female and 40 for a child. (These are only very rough outlines, as am not entirely sure of the exact figures) I know it seems like a bit of extra weight on one pax should compensate for a more "under weight" pax, but understandably it would be impossible to weigh every pax before a flight and there have to be some guidelines.
Also when cargo and pax are loaded, wieght is again impoirtant as depending on how many pax etc depends on how the cargo is loaded..In a nut shell I guess it is all about our safety and making sure we get from A to B in one piece. Several aircrashes have been because of mis management of weight...
Hope this answers some of the questions