It adds vitamins, minerals, and electrolytes to your water to regulate and maintain an optimal PH/alkalinity, as well as to keep fish healthy. It also helps to remove nitrates and phosphates. You're supposed to add it after water changes because you'll remove a lot of the product by taking out water.
It's actually the perfect product to add if your water is from an RO/DI unit, your tap has a low KH, or if your tank has a KH/PH problem. I'll tell you something that actually IS a waste of money, bottled bacteria.