Gun emplacements were positioned on the planet Tatooine by the Desert Sands unit—a division of desert stormtroopers—under the command of Darth Vader circa the Battle of Tatooine, primarily to combat Rebel activity.
During his search for R2-D2 and subsequent journey to Mos Eisley, Luke Skywalker was forced to dodge the continuous fire from these emplacements while piloting his landspeeder.