We once called it a strobe. These days it's a flash. The reason you see serious photographers using an accessory flash is twofold. 1. It's moveable. 2. It's adjustable.
The accessory flash lets you fill in dark areas in an otherwise well lit scene. It lets you create effects with shadows. For me it lets me add just enough light to a shot to make my foreground and background look good together. It ain't cheap but if you want to improve and expand your photography you must break free of the pop-up flash.
In this shot I could expose for the skyline and fill the foreground with my flash.