Clean water is a must when hiking, especially in the back country or on overnight hikes. Ideally you would bring along water from home, but water is heavy and it’s not a smart idea to carry 4 days worth of water with you when doing an overnight back packing trip.
In most cases a water filter will work well with water sources found on the hike. However, filtration will only work on two of the three water contaminants: protozoa and bacteria. Filtering water does not eliminate viruses. In this case you will need to purify your water with either chlorine or iodine. Water purification also kills protozoa and bacteria, but will give an off flavor to the water.
When hiking in the United States, filtering of water from mountain streams will most likely be adequate. But if you find yourself hiking in other countries you may want to consider purifying your water as well. It is also a good idea to carry along some iodine tablets, just in case your filter breaks or clogs.