TL;DR
Increase the usefullness of lamps beyond only being able to see your base better during the night.What ?
Decrease range of base defenses during the night unless lamps are present nearby. Lamps will show a radius circle when in the cursor, similar to turrets. Turrets will have a radius circle for day and one for night. When a tile is within the day radius circle of a turret AND a radius circle of a lamp, the turret will fire at enemies in that tile during the night. Otherwise the turret will only fire at enemies in it's night radius circle.The radius circle of the lamp is bigger than the area that the lamp visually actually lights up, this can be seen as realistic because in real life the light degrades very gradually and will be able to reveal movement from far away. This prevents that the lamp needs to be placed far out in the field.