Spearfighters are the very first military unit in the game. They are trained in a Spearfighter Barracks, which is unlocked after researching the Spears technology. Unlike other units, it takes only a short time (20 seconds) to be trained.
Spearfighters have a defense bonus when standing in bushes and forest terrain. They are especially effective against Fast Units. After the horseman, they have the highest attack and mobility in the Bronze Age. They take additional damage when attacked by heavy-class units.
A player will rely on Spearfighters for quite a while as a result of their versatility and low cost. Furthermore, the Spearfighter, with its short recruitment time, quick healing time, and low training cost, acts as a great cannon fodder. If a quest requires you to delete units, normally the Spearfighter are the best choice in place of more valuable units.
The weird and interesting thing is that if an attached Spearfighter has 1 HP left it will be 'Worst than dead' because it takes 30 seconds to heal 1 HP so it will take 270 seconds to bring it back to full health, but only 20 seconds to train a new Spearfighter. So if you seriously need Spearfighters you can delete the attached ones with 1-3 HP and re-train them.
As with all units, other units can cover their weaknesses. Warriors can be dealt with Slingers, which is bad against Horsemen but you can use Spearfighters on Horsemen and even Mounted Warriors, which is from Iron Age.
At the Bronze Age, Spearfighters can be used to attack Warriors if no other options are available. In later ages, it is less effective using light units against heavy units.
Two spear fighters are the default city defense if no units are selected otherwise. Two unattached spearfighters are provided in the tutorial.
Spears are one of the oldest weapons and can simply be strong sticks with a sharp point. When tribal warfare grew, weapons improved and in the Early Bronze Age they had metal spear tips. In defence, fighters had hardened leather jerkins, helmets made of bronze or leather, and sometimes used toughened leather (Africa) or wooden (Europe) shields.
The Spearfighter seems to have leather armor, a simple bronze helmet and a bronze spear.