The ato will top off existing bodies of water form another body of water.
Why is this great?
Well if you are out of town and someone is taking care of your tank you dont want them fiddling with your water parameters in a freshwater or saltwater aquarium. The ATO will automatically refill the tank once the water hits a certain set point that you set up.
Saves you the the chore of daily having to do it with buckets. I know the A.T.O Auto top off unit has saved me hours of work.
Saves your back.. Enough said on that statement!!!