My answer was perfectly serious.
I think it has something to do with being able to locate the 9 on the dial in the dark.
It is next to the last hole (the 0). Next to that is the stoppery-bit.
I think you were supposed to feel for the stopper, then count to the 9 using first your middle finger in the 0, then your index finger..this only works if you are right handed and dialling with your right hand.
If you are left handed, you would use your index finger as the guide and dial with your middle finger.
Of course, this may all be bunkum and the answer may well be 'to dial the emergency services' or 'because they don't have an index finger'.
Please post the answer when you find out.