The freeze plugs on a car engine block are there to cover the holes that are in the engine block after the casting of the engine block.
During the casting of the engine block sand is used for the mold and the engine block has some sand left in it after it's cast.
So the holes allow the sand to come out of the engine block.
Then the plugs are there to cover the holes.
The freeze plugs also plug off the holes to prevent coolant from draining out of the engine as well.
The freeze plugs in an engine to do not prevent the engine block from cracking during cold weather if you leave water in the engine block.
You must use antifreeze in the radiator and engine block in cold weather otherwise your engine will be frozen and cracked.
There's also drain plugs in the engine block that you unscrew which helps drain down the engine block.