Spears can be thrown using right mouse button, they either hurt someone or stick where they land on; damage depends on speed and material, flight trajectory is ballistic with (unrealistic) drag.
Spears can be used to fight, but are slower and weaker than swords.
Spears can be used to dig, but are slower and weaker and pickaxes.
If you miss the target it's your fault, the spear is fine.
You can grab a spear on the fly (or maybe wait until it falls?).
You can stick a spear into a block, but if you remove the block then the spear falls (obviously).