What is the purpose of this extra PCB pad?
Primary reason is structural to prevent crystal mechanical shock and vibration damage or fatigue.

Since the Xtal is electrically isolated from the metal cylindrical case, it does not need an electrical gnd. It is just like the thru-hole parts with a wide can and very short leads where the can is electrically floating above the PCB surface. 


If 0V grounded, the added sub-pF stray capacitance load might reduce the frequency of x ppm which may be in the range of your average load-cap tolerances.