Good add on for Steripen users
Yeah, you could use a bandanna to pre-filter smut from the water. However, there is a feature on this that ROCKS. You can completely immerse a bottle with the pre-filter intact, open the little air valve, and fill quickly under water. You can also pour water into it for filtering, if you prefer to do it that way.
Quick tip, which Hydro Photon included in their directions. If you put water in it, and it doesn't seem to be filtering through, make sure to pre-wet the filter and give the surface of it a rub with your fingers to fully saturate it. Then it will start passing water.
The mesh is so fine it holds water when dry. At least I found that to be the case. This mates up perfectly with a Gatorade bottle, which is my preferred bottle. You could also do soda/water bottles with the funnel attached.