The layers of ground above the phreatic zone serve as water purification filter that prevents surface pollutants from reaching the water and compromising its quality. Wells are made to access the aquifers and to pump out water.
The depth at which the aquifers can be found differ according to the geology of the location. In general, places near large bodies of water like lakes and rivers have shallow aquifers. A shallow aquifer is convenient in that accessing it is easy. However the proximity of the water to the surface makes more likely to be contaminated by surface pollutants.
Deep aquifers require the drilling of very deep wells but the water is generally safer and has better quality. In many cases, water purification is no longer needed since the water is so deep down that no contaminants from the surface can reach it.
It is important to remember that aquifers do not provide infinite source of water. While precipitation can replenish the water within aquifers, pumping excessive amount of water can cause well to dry up.