Description and uses
A spear is a pole weapon consisting of a wooden shaft with a sharpened steel head, shaped like a triangle or a leaf. The spear is either thrown or thrust.
Broadly speaking, spears were either designed to be kept in hand (thrusting spears), or to be thrown (throwing spears).
A spear is a versatile weapon that can be used for hunting, fishing and warfare. In regions of limited resources this alone was reason enough for the spear to become ubiquitous: the tool that brought down game for meals, slew animals that threatened crops and livestock was also an effective weapon in war. The spear could be used as a melee weapon or as a missile.
The spear was a relatively low cost weapon or tool in comparison with other weapons available in the periods of the spear's greatest use. In pre-industrial societies where metals are expensive, as was the ability or skill to work them, the spear was seen as "cost effective". The amount of steel required for a sword, for example, would be sufficient to make two, three or more spear heads. A spear not only takes less metal, but requires less time, ability and a lower quality of material to manufacture and the resulting weapon still has a potentially lethal effect.
A spear is relatively easy to use. Again in comparison with contemporary weapons in the periods of the spear's widest use, a spear requires less training and practice to be effectively wielded. (Please note that is "effectively" not "expertly".) Modern experiments by re-enactors in the United Kingdom have shown that a group of people could be trained to use spears in an effective shield wall as militia in a few weeks of part time training.
Spears are a cost effective, relatively easy to wield weapon that could be quickly deployed to field relatively large numbers of militia. Effective in that the enemy or prey by the nature of the weapon is kept at a distance. And, finally, effective, in the sense important for all weapons: in the hands of an experienced user it is fast and lethal.