Also have one more situation (but I don't often see many people use this):
Balancing flash and ambient light.
There was a one time I was sending a friend back to China, when I decided to head to the T1 viewing gallery since we had alot of time to spare. Decidded to do some casual shots. It was at night. Since I wanted the planes to be visible, I Ended up shooting at f/4.2 at ISO 1600 with slave flash.
Theoretically I could have used f/1.4 and dropped the ISO to 800, but then DOF would be too thin. I think I used a D80 for this shot.