greek and macedonian hoplite spears, sarissa had a square section counterbalance spike which was also usefull as a secondary weapon if the spearpoint broke or was cut off. it appears that a number of skulls found on ancient battle fields were found with square holes in them, it would have been convenient on the advance to spike your downed enemies as you went by to ensure they stayed down, while still presenting the main spearpoint towards the remaining foe. these SE Asian ones could be convergent evolution.
greek hoplite butt spike (top) & spearhead:
macedonian sarissa butt spike (upper left) & spearhead:
they also could be used to anchor the spear butt in the ground when being charged by cavalry to enhance the hedgehog effect. horses being smarter than their noble armoured riders will seek to avoid colliding with sharp pointy things (in spite of heroic stories and mel gibson movies) and tend to stop suddenly, throwing their riders, which is very embarrassing for the knight in shining armour, also why the british square with it's fixed bayonets was rarely broken by cavalry, the horses objected to being shish-kebabed.
