Not all hot tubs, custom tubs or swim spas are outside in the elements. So it begs the question, since they're either in an enclosure or in the bathroom, do you really need hot tub covers and/or spa covers for your indoor tub? Since the tub won't be catching rainwater, won't be contaminated by leaves or other debris from the yard, etc., is it really important that you spend the extra money and go through the hassle to have a hot tub cover indoors?
Truth be told, the pros recommend that, yes, you do indeed need to have a cover for your hot tub or swim spa even if it is indoors. This especially holds true if you have pets or children since the last thing you want is to have some form of unfortunate accident that simply having a hot tub cover in place can prevent. Also, even though the hot tub or swim spa is indoors, that doesn't mean it isn't possible for the water to become contaminated.
While it may not catch the debris from leaves and other common outdoor sources, that doesn't mean things can't be spilled in them, Another problem is, even if you can trust your pet to only take a dip, most people don't really want to jump in the tub after Fido has enjoyed his break. Since indoor hot tubs and swim spas rarely get used more than their outdoor counterparts, and, granted, since it is indoors, you don't have to have the heavy outdoor cover with the thick insulation as the main goal is to simply keep out accidental entries and spills.
Regardless of whether your hot tub or swim spa is an indoor or outdoor model or whether it's a standard one or custom, protecting yourself, your family and even your pets as well as the hot tub itself means that the pros recommendation of always having a hot tub cover is a recommendation that only makes sense. The investment versus the protection and the potential reward scenarios means a hot tub cover may be your best investment.