You would use external flash to shoot a more detailed shot from a far target or when you want to intensely lighten up a subject for example when taking a portrait.
You do not need to meter light when using a flash since the flash burst is almost impossible to measure.
Flash synchronization is the synchronizing the firing of the flash with the opening of the shutter admitting light to photographic film.
Ettl is a automatic flash mode for a camera.
what happens when you shoot faster then the camera sync speed is that the subject your shooting will be half black and then the other half will be lite with the flash.
Photographers bonce the external flash so that they can control how lite the subject will be.
A slave in photography is when a flash goes off after a press of a button by sensing the light.
If your image comes off to lite or to dark you should lower your shutter speed.