Besides delivering full-spectrum illumination to the subject and being immune to the hazards of bouncing, direct flash is very efficient. That allows rapid recharge and greater range with a smaller unit (although I was using a big one). Regarding my settings, I use f-stop to control DOF, shutter speed to limit motion blur from ambient light, ISO to control ambient exposure, and flash power to expose the subject. Since E-TTL and iTTL (flash auto modes) can be fooled by an excess of black or white clothing, flash compensation is needed to achieve optimal exposure incamera. Shooting RAW @ -2EV and whitepointing in post is an alternative route, but I shoot JPEG. Another example, almost SOOC:
As dmward says, people want clear pictures of their faces. You get 1 second or less to make those shots before the moment passes or the spontaneous expressions glaze over. Hauling around an off-camera lighting rig is out of the question!
Canon, Nikon, Contax, Leica, Sony, Profoto.