Regular testing is important because they can identify existing problems and ensure water is suitable for the intended use, especially if used for drinking by humans and animals.
The only way to determine if your drinking water is safe is by having it tested at a certified laboratory. Harmful bacteria, parasites, and viruses are invisible to the naked eye. Water that looks and tastes good may not necessarily be safe to drink. These microbes can exist in surface and groundwater supplies, and can cause immediate sickness in humans if not properly treated.