Quote from zboy2854A:
There's one other major reason one might prefer to trade the futures instead of cash market, and that is interest rates. Depending on what currency pair you wish to trade, you may end up having to pay interest to hold a position in the cash market, whereas no matter what pair you trade in the futures market, you are not liable for any interest payments.
Of course, this only applies if you are holding the position for longer than a day, and assumes you're trading a pair that would require you to pay the interest differential. In cases where you would benefit by receiving the interest differential (aka carry trade), obviously you'd want to stick with the cash market.