The most common theory states that the fifth pocket is a 'watchpocket' - designed for holding a pocket watch, the chain of which can be clipped onto a belt loop. Note Webby's first entry under fob which specifically references this idea, "a little pocket for a watch." This makes the phrase "fob pocket" which is sometimes seen a bit redundant. (For an example of that usage, one that incidentally shows some skin tight "Ladies' Breeches", see http://www.tallyho.ie/ladiesgenb.htm.)
I had previously thought it was for carrying zippo lighters, which fit in there quite nicely with most makes of jeans.
I myself keep my Canadian $1 and $2 coins in there instead, but then I have to contort myself at the checkout to squeeze the little blighters out again.