Pulldown on optocoupler's input side
Given the resistance ratio of 100:1 which represents about 1000:1 in current (10mA in operating current for the LED vs. 10uA of bleeding leakage, EMI or whatever at the ~1V where an IR LED begins to conduct) it doesn't look particularly useful. 

If you're expecting that much crud on the input the LED should have reverse protection and it doesn't. 

If they expect the '10V' input line to go right to zero when opened up, then it might have some function, but that would depend on something off the page.