Accurate time-base / clock: In order to create the various gate / timing signals within the frequency counter an accurate timebase or clock is required.
This is typically is a crystal oscillator and in high quality test instruments it will be an oven controlled crystal oscillator.
In many instruments, there will be the capability to use a better quality external oscillator, or to use the frequency counter oscillator for other instruments.
This is also beneficial when it is necessary to lock a number of instruments to the same standard.