I recently saw a bicycle touring documentary that a bunch of Seattlites made going from Beijing to Shanghai-- and almost all of the old bikes being ridden in the countryside by the Chinese had these truss rods. I wonder what on earth they are for? I don't see any possibility of suspension from them here.
As you might imagine the bicycle in rural China (and many other countries) isn't for fun, is a mode of transport and a real workhorse. So when they load them up with 150 kgs of rice, or whatever, and then ride them down bumpy roads its very hard on those front forks, sticking way out there with that nice bend in them and their slack angles. So the truss rods reinforce the fork so it doesn't fold up under load.
Now I think they have been relegated to the aesthetic on many bikes, but they have their roots from the working bicycle.