As they can only attack air units, the AI will use them so, and that makes air units an attack priority over ground units. Due to the extreme power of air units, that is a necessity. These should generally be built scattered around the base to ensure they can always attack the opponent. You cannot choose the target for them, nor can they move. The portable equivalent is the RPG Trooper, which has an anti-air attack, but is considerably weaker and places priority in ground units.