There is a lot of arguement as to if they are needed or not. In a "Perfect" engineered design you probably don't need one. But, most pools are far from perfect and having one will improve circulation and help maintain a much more steady temperature. Remember that heat rises, so the bottom of the pool will tend to be much colder. By pulling the cold water off the bottom you mix it better with the pump and eliminate thermal gradients in the pool, not to mention circulating chemicals better.

If there were no reason for them you can be certain contractors would not install them as they cost money.