Answer:
Water often became a breeding ground for various types of insects and Bacteria's. If Water is stored in a certain place for a long period of time, that water can be uncontaminated without we even realizing it.
If we use that water for consumptions, we can put those small insects or bacteria's into our body and it can create detrimental effects to our health.
So, unless we developed a proper sanitary measures to maintain the hygiene of the water, we better not store the water in our house for a long period of time.