Without a doubt, the most essential element to human survival is water. While humans can live for two months without food, they can only go without water for a few days [source: Scientific American]. Carrying a water purifier on an outdoor excursion could save your life.
Water filters capture the tiny bugs that live in water so that you don't drink them. If you did, you might wind up with diarrhea, vomiting or much worse. Water filters come with different pore sizes -- the smaller its pores, the more bad stuff the filter is going to catch. A water filter with 0.4 micron pores is recommended for camping because it will filter out bacteria, in addition to larvae, parasitic eggs and protozoa. Some water filters include iodine to also kill viruses in the water.
If you want to go high-tech, there are battery-powered purifiers on the market that use UV rays to kill bacteria. But low-tech filters do the trick without batteries.