Total Alkalinity is a pH buffer that can play a large role in how easy it is to maintain the pH in the hot tub. A hot tub with too high Total Alkalinity will cause the pH to rise, while a hot tub with too low alkalinity will cause the pH to fall. When you first fill your hot tub, it may be difficult to maintain proper pH until you are able to get the Total Alkalinity within range.
Total Hardness is also an important part of water balance. Water with too high Total Hardness is more susceptible to scaling. Water too low in Total Hardness is more prone to foaming.
Finally, chlorine is an important part of water balance, not only for killing bacteria and viruses, but also for oxidizing sweat and other contaminants in the water and maintaining crystal clear water.