I find the Fn button to be extremely useful. I have set it using Function Menu Set (gearwheel icon menu tab, 6 sub-tab) to:
First row: Quality, Aspect Ratio, Picture Effect, DRO/Auto HDR, Creative Style, Grid Line
Second row: Metering Mode, Focus Area, Peaking Level, Zebra, Flash Mode, Flash Comp.
It provides an easy way to set key functions on the VF using the wheels.