The butt pocket seems to be a major advancement in Men's fashion. (claim it as a fashion and not a necessary based on the following arguments). Though I do not know the history of this significant development, I am curious of its development and acceptance as in the Male community to house the "Wallet" in it.
Its astonishing that the Butt pocket mainly serves for the Wallet and Comb (mainly Indians) and sometimes bits of Paper. The introduction of the butt pocket (BP from now) seems to have come out of the need to remove irritation from the thighs. One can understand this phenomenon trying to walk with a wallet in the front pockets of the pants. While growing up its a sign that you have matured i.e you have grown from a child to an adult and you can handle the wallet in the Butt (its a style statement as well among teenage boys) and dont need to protect it in your thigh pockets.
But apart from this situation the BP/wallet scenario is mainly negative. It is uncomfortable while sitting, inaccessible (wallet) in crampy seats, and feels like you are paying the bill from the Butt (looks and feels derogatory). Security wise it is counter intuitive as well.
Another interesting fact is the pentration of other products into the Butt pocket. Later advents (mobile phones, ipods, PSPs) never got into the Butt, I mean into the BP. These are still largely reserved for the thigh pockets. Even the rumors of wireless signals harming genital parts could not move the mobile phone from front to the back (i speak of majority here)... leaving the Butt pocket exclusive to Wallets and Combs (in case of Indians)
So, weighing the positives vs negatives it looks like mostly out of Fashion rather than purpose ?? isnt it?
ps:- I love having pockets in pants (not in the shirt though). Imagine otherwise carrying everything in a Man purse.. phewwwww.. so many more animals would be sacrificed and ofcourse you would compete with the ladies in one more arena (who has the better bag)