Well unless you have a Kinetico or big box store brand softener; such as Kenmore, GE, Whirlpool, North Star, mortonsalt.com. They and Kinetico don't work well with any invisible dirt build up.
The vast majority of softeners do not need a prefilter. Their control valves don't gag on invisible dirt build up and, having a prefilter can cause improper backwashing because they reduce the flow and pressure to the softener, which ruins resins used to remove iron.
Does that make sense to you?