but surely the built in flash could handle things like that?
No.
Well... I suppose it might help, but the built-in flash is crud. You really want to use a better flash.
See, the more you move the flash away from the lens axis, the better off you are. It prevents red-eye, it gives you a more natural look and it gives you a lot more options. You can add diffusers, bounce the light to avoid harsh highlights and shadows, even focus the light on a specific area. You can't do any of the that with the on-camera flash.
For example: If you are taking a picture of a man who is standing with the light coming from his left and behind him (like it is looking over his shoulder) the right side of his face is going to be in deep shadow.
Using the on-board flash might bring out that shadow, but it will also blow out any highlights on the left side of his face.
You can use the on-board flash to compensate by focusing on the man and then, while holding the shutter button down half way, shifting your aim to the right, but that only works if there is nothing on that side that will distract from the subject and only if he is looking that direction to begin with. Even so, the on-board flash has a wide arc and you will have to shift far right to correctly expose the darker parts of the picture.
An aftermarket 'flashgun' (as you Brits call it) will give you more options. You can focus an aftermarket flash on his right side, or bounce it off an object to his right, and it will bring up the detail on that side without overwhelming the natural light on his left. You can use a diffuser to add light to the foreground that will bring up the ambient light on the shadowed side. The ETTL function on a good flash will allow the camera to correctly gauge the strength of the flash more accurately than the puny on-camera light.
Hope that helps a bit.
