It works like a crossbow, uses the same ammunition, but has longer range (1 tile more, 6 sqm). The extra range can be very useful when hunting dragons and they flee in low hp, for example. It can however also be a hindrance due to the extra 55 ounces lost on cap for bolts or other necessities. If you don't need the range it is recommended you use the crossbow instead. Value increased with the Winter update 2007 because of the reduced spear range.