What is the purpose of balancing tank in reflection pool?

If you have a deck level pool or spa (water level with the top of the pool, that usually overflows into a grating around the pool) when people get into the pool their body displaces water, this water is stored in the Balance tank until the people get out and the water is pumped out of the balance tank and back into the pool. A skimmer pool that has a lower water level than the top of the pool does not need a balance tank as the displacement is handled by the water level rising. Deck level pools and spas are considered more aesthetically pleasing.