There are only TWO reasons that I can think of to have stacks:

1:  Your 'rig' is an 18 wheeler  (duh)
2:  To bring the exhaust up to clear very deep water crossings (and this is stretching it)

Although, reason 2 is rarely applicable due to the fact that most do not waterproof the engine.

      wow your an idiot i will agree that those stack are pretty bad however stacks were used for keeping the exhaust high enough so that it didnt go straight into the stock trailer they were hauling  thats why you see real cowboys with really tall stacks the other are just for show some look good i think you can have your opinion others i think we all agree look like