For some reason, all the Duplo bricks that exist out there have holes in their studs. I haven't seen a Duplo brick that doesn't have holes in the studs, and I doubt such thing even exists.
System bricks can actually connect to Duplo bricks with these holes, but is that the only reason for the holes? Is there is a Duplo brick that doesn't have holes in its studs (if yes then please point it out in the comments or in your answer)?
What are the other reasons for Duplo bricks to have holes in their studs, if any?