We are not historians and information does vary on the web, but it seems like the original version of roulette (which originates from Europe) actually had two zeroes to begin with. This was then changed into a single zero to attract more players. As roulette reached America the single zero tables never made it. At one point in time a third zero in the form of an American eagle was even introduced. To answer the question more literally, double zeroes simply exist to give the house a better edge.