What are the benefits of currency trading
All the derivative instruments are introduced with the idea of hedging and speculating.
You are expecting some inward in dollars after few month and your view is dollar will depreciate by then. So you have actually short USDINR contract and hedge your loss with the future contract.
Well, this is just an example, trading in currency is one more asset class to speculate and trade.
Trading in general-
It gives you opportunity to earn money
It gives you more opportunities to loose money than you can earn
First of all, it is a big trading market with large liquidity. If you are good at speculating the currency movements, then it is a very good source for income generation.