To my understanding other members may be more familiar with its role,
When your speculation currency it's like the futures market your hedging for something to happen, so lets say USD>AUD in value, i'm speculating that the aussie dollar is going above parody against the green back (USD) in the near future, i'm going to horde as much AUD as i can, trading it for in a general sense the USD, now when you get enough people on the same boat, (financial markets all about trends in a sense then) you start to tip the scales and the value of the AUD goes up against the green back. This can work with other scenarios with more currencies but in a general sense it's like say i'm hoping for this to happen and i want to be in with a slice of the action when it does.