It depends upon its polarity. If forward biased, the first thought that comes to my mind is that it cancels out the forward voltage drop of the clamping diode by dropping the same amount of voltage as the clamp thus ensuring the circuit input can never exceed the power rail.
Yes it appears to be a simple reverse polarity protector. Without it the fuse would blow if the polarity was accidentally reversed. With it in there if the polarity is reversed nothing happens so the operator knows they have got it backwards, and after correcting the polarity then it suddenly works fine without having to change the fuse.