Functionality of this diode configuration?
Yes, it is over-voltage and an under-voltage protection.

This method assumes that the power/ground are low enough impedance to cope with excessive voltage. The 10K resistor limits the current. 

Anything negative gets routed to ground but still gives a small negative voltage on the input (The diode voltage drop).

Anything above 3V3 gets routed to the supply rail but still gives a small voltage above 3.3Volts on the input (The diode voltage drop).




  Can you elaborate on the idea of power/ground impedances being low enough to deal with excessive voltage? I guess I'm not understanding that part..


To start off: the 10K series resistor is in this case a savior as it will limit the current. However I have often seen this same protection circuit with no such series resistor.

With an external high voltage you get energy coming through the diode arriving at your supply. Your supply now has to get rid of that energy otherwise the supply rail goes up.   

Some power supplies can only supply current not sink it. The excess energy has to be taken up by the circuit itself. If you have a very low-power circuit e.g. something which uses uA. then the impedance will be high and your supply rail will go up. 

There is also distinct possibility that the over-voltage is a short 'spike'. You then get a high frequency pulse injected on your supply rail. You can imagine that, that is not ideal.

The same holds for the ground. The energy has to go back through the ground return wire. A noise spike might still cause your ground to bounce.