"if you are using the forecast wind, why not do it on the ground?"
A very good question, but I was taught to use the whizwheel in the cockpit during the NFT which used to involve a surprise diversion. Since you didn't know the diversion track in advance, you couldn't work it out on the ground, but you did set up the whizwheel with the forecast wind.
I now would not dream of trying to fiddle with a whizwheel in the cockpit although I am quite happy to use one on the ground. When in the air, I do all the drift calculations in my head or using the DI if the aircraft is fitted with one. (Some of the aircraft I fly don't have DI's fitted). This is quite accurate enough for VFR navigation, and for IFR you have enough instruments to fine-tune the drift calculation.
I wonder if anyone is still taught to use the whizwheel during the flight? Even at that time I thought it was too distracting and affected the time spent looking out. For a real diversion (possibly at low-level and in bad vis) I would think it could be deadly to divert your attention to using a whizwheel.