Why are the strings closed?
To increase the mass per cm of length, since the metallic wound is heavier than the casing or nylon.
What does the metallic wound have to do with the Aquila Red Series strings?
That the Aquila Red Series ukulele strings carry the wound inside, because they have copper particles inside the strings. In this way you get a higher density (weight per cm of rope) without having to twist the strings. Not only is it cheaper, but the noise of rubbing the fingers with the rope is avoided.
Red Series are strings with a smaller diameter and a slightly higher tension than normal strings. The ukuleles projects the sound better from these strings and the tuning is improved since they have a lower elasticity.