Working with multiple Speedlites gives you fantastic creative control over your lighting. For example, with the main light away from the camera you can provide flattering side-lighting to reveal your subject's shape while other units fill in the shadows, add backlighting or illuminate the background.
It has been possible to work with two or more flash units almost since the day they were invented. However, the problem has always been getting the exposure correct. Just synchronising the flash outputs is not enough – you also need to control each output independently to give the correct amount of light for the subject.
Canon's wireless flash system can handle this for you, making it easy to create the images you want. You just set up the Speedlites and press the camera's shutter release button. A series of pre-flashes will be fired and the system automatically calculates the output required from each flash unit. It then fires all the units simultaneously to give what should be a correctly-exposed, well-balanced flash-lit image.