Re: function of pll
The key, as some have alluded to, is the phase noise of the VCO.
If you have a lot of reference noise, you can use a low phase noise VCO to clean it up. You would use a SAW or CRYSTAL VCXO and a narrowband loop filter, maybe set at 100 Hz. Then the VCO acts as a flywheel to the perterbations of the reference, but in a slow time frame the reference keeps the VCO on frequency.
If you try to use a low Q VCO, you will find that the phase noise in your VCO might be higher than the reference phase noise, so the PLL no longer cleans up the phase jitter, no matter what the loop bandwidth.