As I see it, it is used to load the oscillator to 50R, so that measurements are done with the oscillator loaded at 50R.
Then any voltage measurements with the 1M scope probe will actually refer to 50R.
Is that so confusing?
What I am saying is that if a commercial module is to be measured, there might be no need to put the 50R feed through resistor, because these modules are already matched to 50R, in other words their output impedances are 50R. So paraleling another 50R (the feed through) will give voltages at 25R load, not 50R.