Quote from AAAintheBeltway:
You actually raise an interesting question, at least from an historical standpoint. Spot FX, as is currently available from various brokers, is a relatively recent concept. Back in the day, ie 1980's and 90's, FX trading was the province of banks. You had to be able to swing a pretty big line to participate. The CME had hit a big home run with S&P futures, so it was natural to look for something else, hence currnecy futures were born. Unlike FX, in which the "larger" currency is always the numerator, CME currency futures were all priced in dollars, just like beans, and had a fixed contract size, just like every other commodity.
Since futures have an expiration date, they are priced somewhat differently than spot. In spot, interest rate differentials are a key factor. You can earn a nice spread by shorting a low yielder and going long a high yielder, provided the relative values stay close. And of course, that spread can be multiplied many times by leverage. With futures, the rate differential is already priced into the contract value. Of course, you have to hold the contract until expiry to realize it all. And leverage is fixed by exchange margin requirements.
More recently, another product has been introduced to allow currency trading for smaller players, FX ETFs. They trade exactly like stocks, so you don't need to have a futures account.