Maybe they take up less space? I don’t know.
But I really like them. Better than rectangular ones actually. I always stick my tongue through the hole. =]
I think the ice machines use a cylindrical extruder to make the “cubes.” They have to make lots of cubes fast, so the water passes through a super cooled tube then drops into a container.
Eambos is right.. With the cylinder and the hole they can make them faster. It allows them to be cooled from the outside and the inside at the same time.