Yes, this gap is important. As the ice slowly forms the impurities in the water are forced through the small holes in the base of the mould so leaving the cubes clear and the cloudy impurities in the ice in the reservoir below.
Somehow I got a little confused when ordering and bought the spears instead of the cubes mould. Is there any way to buy just the cube mould without the box as I can share the box with both?