The Expert Advisor is a very sophisticated trading software tool. Actually, it is a robot.
Most forex traders fail because they fall prey to the human emotions of fear and greed. However, trading with an Expert Advisor removes these emotions so that trading can be executed on a purely logical basis.
So often a trader will hold on to a trade in an effort to grab that last pip of profit even when his/her logical mind says to exit. Conversely, many times a trade is exited prematurely out of fear, leaving profits on the table. The Expert Advisor has a plan, sticks to it without regard to outside influences, and does this 24 hours a day during market hours.
The Expert Advisor watches the market for you and will execute trades based upon predefined parameters. Unlike a human, it is also capable of monitoring indicators, support and resistance levels, and many other factors in multiple timeframes and making immediate decisions.