The Harpoon Gun was fired from the Rebel Alliance's Snowspeeder and is commonly used to pull cargo. In Battlefront I and II the harpoon gun was used to "trip" the AT-ATs, which was based on a scene from Star Wars The Empire Strikes Back. Some Harpoon Gun's had a blaster cannon built in, however this cannon was not very good.
The harpoon gun is very useful in Star Wars Battlefront I because the Empire's AT-ATs count as command posts, so bringing them down can deplete their reinforcements. Generally, the harpoon was fired at the joint on the AT-AT between its leg and its foot. The snowspeeder would then circle around the AT-AT until it collapsed and was therefore destroyed. The co-pilot can use the harpoon gun, and it is located on the back of the snowspeeder.