Why bother to boil the water at all you ask? Well if you are out in the wilderness and stumble upon a stream of water you would actually be taking a risk by drinking from it without sterilising first. Naturally water is a host to many dangerous pathogens, bacteria and parasites which can cause you serious illness or even death!
Boiling your water is the best way to ensure it is suitable to drink, filtering devices and chemical purification products are not as effective. Boiling your water will kill all kinds of micro-organisms such as bacteria, parasites and viruses.
The fact of the matter is by the time your water has reached boiling point (212°F / 100°C) all pathogens have already been killed! As a precautionary measure you may wish to boil for a full minute just to be safe. The time it takes for your water to reach a boil and then cool down is more than enough time to kill all harmful organisms that could make you sick.
To be safe you may wish to follow the advice of the US Environmental Protection Agency which recommends boiling for 1 full minute, or 3 minutes if you are 2000 meters above ground level:
This issue can finally be put to rest! To kill all harmful pathogens and purify a natural untreated source of drinking water it should be boiled for 1 full minute unless you are conserving fuel or at high altitudes.