The purpose of testing your water supply (sometimes called sampling) is to see if the levels of bugs, chemicals and metals in it are above the limits in the regulations
The presence of bugs or substances above the required limits is a danger to your health.
If you have a large, or commercial, private water supply, you must have at least one sample taken for testing by your local council every year.
This is done to show that your supply is 'wholesome' and complies with the legislation (you may be charged for this).
If you have a smaller, non-commercial supply, testing isn't compulsory. But you can ask your local council to test, and risk assess, your supply for you.
This will give you a clear idea of the potential risks. They may charge you for this.