Rashi on Genesis
והיו למאורות AND LET THEM BE FOR LIGHTS — They shall serve also this purpose, namely, to give light to the world.
Rashbam on Genesis
והיו למאורות, they should provide light for the physical universe.
Sforno on Genesis
להאיר על הארץ, they should dispense their light at a time when it is beneficial for the inhabitants of the earth.