To answer that question I would ask 'Why drink freshly squeezed Orange juice, when there is concentrate?' Or 'why use fresh veg when you can buy frozen?'
The answer is that whilst bottled water is checked less, it has been filtered through rock and clay for thousands of years and is bottled as soon as it comes out of the ground. It has never met contamination, and crucially has never been through the human body before it is drank by the drinker.
Tap water, whilst looking similar, is devoid of the minerals and purity contained in mineral and spring waters.
Most relevant however is that, due to the fact Tap water has to be heavily chemically cleaned due to its origin (sewers etc..) the drinker is subject to the setbacks when a new type of sbstance is found - such as the traces of birth control, cancer treatment and anti depressant drugs - which cannot be removed by standard chemicals.
So the question should be: what is cleaner - water from a mountain spring, or water chemically altered after 18 recycles through the digestive system?
http://www.independent.co.uk/news/science/test s-for-drugs-in-tap
I hope this helped