They were likely used for shipping or trade and would have been stored in something similar to a modern day egg-crate which kept them from bumping together or falling over and breaking. Similarly they likely would have been kept on sand or dirt floors where the bottom few inches were covered. The points were probably added to the bottom exterior, and in most cases the inside of the vessel would have been round without the cylindrical point for structural integrity.

my craft is ancient greek pottery, Apparently the pointed bottoms were so for storage in the hulls of ships.