The snowspeeder has a gunner's seat for another player to utilize, whether it be an NPC or another person. The gunner has control of the harpoon cannon, which is used to tie up the AT-AT's legs.
You can fire the harpoon cannon without a gunner, but that requires switching from the pilot's view to the gunner's view while still controlling the flight of the snowspeeder. The switch is accomplished by using the mouse wheel or pressing the second function button, I can't remember if it's by default F or G, but it's one of those. Perhaps there is an easier way to fire the cannon, but I haven't encountered it yet.