Hazard lights are designed and should be used when your vehicle poses a hazard to other drivers. They are not supposed to be a warning system about other hazards one might encounter independent from your vehicle.
- Driving especially slow with a large cargo load? Yes
- Picked up a flat and trying to limp to the next service station? Yes
- Parked on the side of the road or shoulder? Yes
- Stopping downtown to pick someone up? Yes
- Anytime your specific vehicle presents an obstruction or danger that is uncommon to the other vehicles in the area? Yes
Outside of that, I could see an argument to increase visibility in certain circumstances where vis is less than a couple of car lengths (i.e., those random West Texas sandstorms that pop up out of nowhere and you suddenly cant see a foot in front of your face, nor can you slam on the breaks and stop because a tanker is going to kill you, flash fog, whiteout, etc.).