The dangers of placing warning triangles

Today again I saw a guy wandering down the freeway placing his little warning triangles behind his car (as he is required to do by law here in Spain). I can appreciate the fact that this is a good thing to do on a winding country road with limited visibility, but walking down the autopista full of speeding semitrailers seems downright crazy. Especially since the point of traffic laws is generally to enhance safety.

If there was any danger that a another car wouldn't see your car and crash into you, what do you think is the danger of a car not seeing the human walking down the road? Personally, I'd just put on my hazard lights and wait for the police to show up.