It's better to expose the water to sunlight, not only to breathe but also to have the UV in sunlight break down some of the chlorine because when it does it produces hydroxyl free radicals that are very powerful oxidizers so can help clear the pool faster. It's the chlorine that gets in cells and kills the algae, but having some of the chlorine lost to sunlight is a good thing in this case, just not too much. If you've got CYA in the water, then you won't lose too much chlorine too quickly.