Well, it makes some difference inasmuch as the filter removes any major particles, dead insects and that kind of crap. Filtering may not be necessary if you get water from a reasonably clean source. If your only source of water is a stagnant pool though, you'd better make sure to filter first. That is, unless you don't mind drinking stuff that tastes like boiled mud, cattle [censored] and diarrhoea with a bunch of drowned flies and ants to spice it up...