There have been several articles in the Press lately about the health hazards of drinking water stored in plastic bottles. This is because as the plastic in the bottle warms up (specially when left in the car) the plasticisers also warm up and are released into the water. These are then ingested while you drink. The ingestion of plasticisers has been linked to health problems including cancer.
When you already have a water filter at home simply refill everytime you leave the house and avoid purchasing uneccessary bottled water.
An individual may consume 100s of plastic bottles a year - this all leads to a great deal of waste that can be avoided.