All bacteria require some amount of water. Some can live in evaporating salt water or moist pastes, but completely dry foods such as a bag of sugar are free from bacterial attack. Dried meat or fruit are free from attack by most bacteria, but those foods may be attacked when the humidity increases. However, attack of damp dried foods is usually by fungi rather than bacteria. While scientists use distilled water in vitamin studies, in instances where hard water might affect pH of the medium, or in situations where the water might contain interferring substances, many bacteriologists prefer tap water for making medium to save money or supply trace minerals.
Water is the main ingredient in any medium.
I usually prefer tap water because it contains some minerals and is cheap. Advanced experiments may require distilled water when you are testing exactly which minerals are reguired by an organism. Since most commercial media contain calcium and other ingredients found in tapwater, it is seldom neccessary to buy distilled water. When growing protozoa or animal cells, the chlorine in tap water may kill the animals. If so, you can let the tapwater age open to the air so the chlorine leaves, or do as many professionals do, use water from a well and call it spring water as bottled water companies do.