On big aircraft (see Falcon 20, if I remember) the fence is to reduce span-wise flow
Well the a/c in the original photo is a G/A Piper twin with non-swept wings, indeed if you look at the "fence" in question it measures just a few cm of chord so is no aero value whatsoever!! Not to confused with genuine wing fences especially on 1st/2nd generation swept wing jets which do indeed help to reduce span wise flow. Another solution used was the "dogtooth" or "sawtooth" leading edge.
