ol 55 is correct. Their only purpose is to support the casting core. Without them, there would be no way to cast the inside passages for cooling an engine block.
As far a freeze protection, when water freezes inside a block, the place that gets the most pressure and where the crack happens is the LAST place that the water freezes. The typical crack in a Stovebolt block is along the left side of the engine at the lowest point of the water jacket, not at the core plugs. Water in a lake freezes from the top down, and likely the same thing happens in an engine. It's a property of the density of water as the temperature changes, and the reason that icebergs float.
