Do you have any idea what a spa cover is? The name of the object is already a dead giveaway. It certainly implies that it’s something that’s used to cover the top of a spa or hot tub. So basically, the question should be, “What is the purpose of spa covers?”
Spa covers are generally used to keep the temperature setting of the water by not allowing heat to escape when the tub is idle or not in use. Because of this, the amount of energy used by the heating mechanism is reduced. Spa covers also help in extending the life of the heating mechanism.
Of course, spa covers also help in keeping the hot tub water clean. After all, who wants to relax in a dirty hot tub, right? But more than that, they help prevent the loss of chemicals through evaporation, thus maintenance of these chemicals become easier and less frequent.