Why are there always holes in Duplo bricks?
Plastic molds are designed to have an even thickness wherever possible (I think it helps with cooling), so a having a hole in the stud made sense to keep the brick's walls consistently thin. The hole could have been placed on the underside of the brick, but putting it on top had the obvious advantage of being compatibile with standard bricks.
Interestingly, DUPLO baseplates have their holes on the underside rather than the top. I believe this is because baseplates are vacuum formed instead of injection molded.