Terry, the idea is to make the vehicle visible when it otherwise would not be. The point isn't to get the attention of other drivers, which as you said would be more effective either flashing high beams or using the horn. Any time near sunrise or sunset when the sun is behind you, many oncoming drivers can't see well into the sun, and having *any* lights on can help them spot you. And of course in rain or fog.
Some states require lights / headlights on when the windshield wipers are on. I'm shocked at the vast number of people in Idaho who have all lights OFF in thick fog... one genius even flipped me off when I flashed my lights at him. This was pea-soup fog with visibility of maybe a couple hundred feet. Wish I had a dashcam to capture that.
BTW, I use the fogs as "manual DRL's" in all our cars (124, 210, 211, LR4) because I generally have higher-wattage headlight low beams, with short bulb life spans. So rather than burn up fancy/expensive bulbs, I use the fogs... equally effective IMO, and the fog bulbs last a long time.