It is used in (outdoor mostly) pools for the purpose of raising the pH, so you cannot add too much of it or you will negatively alter the chemical balance (and the availability of the chlorine sanitizer) of the pool.
In most areas where the rain is acidic (="acid rain") and the pool pH is reduced every time it rains, it is used in place of the highly marked-up product they sell you in pool stores labeled as "pH up" to restore the alkalinity of the pool.
OK pool owners, now you are equiped with the knowlegde to start saving $$$!

Not to mention it's one less (at least) product you now need to have around the home.