Why do some non-precision approaches have stepdown fixes after the final approach fix?
Take a look at KPRB VOR Rwy 19:



There is a stepdown fix after the final approach fix. Now look at the terrain around the airport. You can see a bunch of small hills around the FAF. In order to provide adequate terrain separation and get you low enough to land, they need a stepdown. You can also see this on lots of non-precision approaches where the approach minimum is lower if you can identify a point past the FAF. 

If you are interested in how they decide to include a stepdown fix, check out FAA Order 8260.3C TERPS, Section 2-6. Final Approach. Lots of formulas but the gist of it is they have a required obstacle clearance that must be met and maximum descent angles. Sometimes they need a stepdown fix in order to make the approach work.