1979 Ford F150 Using the timing light... What's the adjustm

I am trying to correct the timing on my 351M ford engine. I know where the timing marks are and how to adjust timing by turning the distributor, but my timing gun has a knob on the back you can twist to select anywhere from 0 degrees to 50 degrees (if I remember correctly). How do I use this? I speculate that you view the actual timing with the gun set to 0 degrees (TDC) then adjust the dist. till you get the timing you desire. So why would I need this knob?