When you're a hundred miles from civilization or in the middle of a natural disaster, you can't count on being able to quickly get your hands on replacement water filters. While camping, you would be faced with the prospect of abandoning your vacation site for hours. In an emergency, you might literally be unable to get to a store that sells filters due to high flood waters, icy conditions, downed power lines, etc. So make sure you have some replacement filters on hand, ready to go just in case you really need one at the wrong time.