Cacti grow in the desert, which is a very hot climate. Not the best for both gaining and maintaining water supplies, due to the fact that water becomes vapour very quickly in conditions such as a desert (unless we're talking about polar deserts).
Keeping their stomata closed during the day stops water vapour from escaping their cells. While this does inhibit growth and stop new water molecules from entering, it allows the cactus to maintain a healthy amount of water within itself.