Why is there a resistor in this reverse-polarity-protection circuit?
I'm betting on this as the reason:


In an industrial environment you will want the design to be somewhat ruggedized, and that's what this circuit does.

It protects the input from


Reverse-voltage (via BYM10-1000)
Overvoltage (via 56\$\Omega\$ and Z2SMB36)
High-voltage transients (via 56\$\Omega\$ and 2.2\$\mu\$F)