I think it is all for looks. Occasionally you will see a practical one. Like a service truck to get the exhaust away from you while you work with the truck running for the crane or other accessories.
My guess is that they don't have mufflers. However, you can get mufflers to fit inside the stacks.
I don't much like the whole look / idea much myself. It makes the bed less usefully.
I bought a 73 Chevy 1 ton with a 4-53T Detroit a few years ago, drove it home from Colorado. As soon as I got home, within 10 minutes, I had the two vertical stacks that were outside the bed removed and in the junk pile. From the junk pile I retrieved a bend of 4" pipe to use so the exhaust pointed out behind the cab. Much less tacky looking.
