External Flashes have many advantages over on-camera flashes; not only is an external flash much more powerful than a small‚on-camera flash, but it also has a tilt-able head so that you can bounce it. Bouncing a flash is a great way to soften the light since direct flash is quite harsh.
An external flash gives you far more control over how you want the scene lit — the built in flash on your camera is usually only good enough to light a subject from a short distance and the light from the flash can only be used from one plane. This is another area where an external flash shines 
External flashes can be taken off camera. This allows the photographer to creatively light a subject from different angles. You’ll need a way to trigger the flash and there are several good methods depending on your budget. The cheapest way (20 bucks or less) is with a sync cord but a better way is a wireless triggering system like the Pocket Wizard.
So if your budget can cope, an external flash is a prime investment and a stepping stone to getting you closer to the ‘advanced photographer’ status. For additional info check this link from our photography forum.
