When the source of your water is a government supply that has been properly treated then a water distiller is not absolutely required. It may remove some contaminants but the original water source probably safe enough to drink. If you are concerned about water source in the water a distiller will completely remove this substance.
When a person wishes to improve the taste of their water and remove minerals a distiller can be a good choice. They will get chemically pure water.
A water distiller will make very clean and free of bacteria water that is suitable for a variety of purposes including cooking with, drinking and many other things. A distillation unit will gradually remove all traces of water contamination, minerals and metal as well as a variety of chemicals that can oftentimes be found in tap water
If the water can easily be seen to have contaminants inside of it then the distillation process will get rid of most of it but it is still not a good idea to drink the water just to be on the safe side. These systems are not designed to fully covert heavily contaminated water from polluted lakes into 100% pure water. What they are designed to do is take reasonably clean water and convert it into 100% chemically pure H2O.