If you run one of those things, the add on pre filter is needed to bring the filter up to acceptable safe filtration levels.
On a paper filter, they can be a big help if out in the desert for extending filter life. You can take them off and clean them and keep going. Some folks out in the desert who have to make extended runs on silty dirt roads will make an extra wrap over those as well with some old ***** hose scavanged from resident female. A layer or two of that catches quite a bit and you just peel it off and pitch it and wrap another on. Dang women always go through enough of those things to keep you in a steady supply
