@Bobby D ^^That^^ is the biggest reason, but with a slight twist:
If it's cooler/cold outside and the fan comes on "all the time", then it'll either take hours or never for the engine to properly warm up because the oil is too cold. Also, cold running like that invites water condensation in the crankcase and elsewhere. That is evident by that beige slimy stuff inside of the oil filler cap or clinging to the upper half of the dipstick or rampant in the valve covers (yuck 
). That's coagulated water/oil mix that hasn't been able to boil/evaporate the water out. And we all know that water and iron don't like each other and the water always wins.
So, let the engine oil temp climb up to 180 - 205-ish and have a happy, non-slimy engine.
Now, about that "Will it take long to bring the heat down if it comes on at 180 vs constantly cooling it?"
With a decent, fan assisted external cooler I would expect that the fan comes on at 180 and starts cooling, slowing the temp rise more and more til it levels out at whatever - 200? 205? 210? Whatever your car likes under the present conditions. This whole process takes a couple of minutes, depending on ambient temp and how you're driving - it happens faster at higher revs. Without the fan, it just keeps rising (as you saw) to the best effect of the two coolers (internal/external) til it levels off 20 - 35 degrees higher (or more).
Just remember, as you're gazing at your Dakota gauges, that an aircooled engine under varying loads (speed changes, uphill/downhill, outside temps, etc) is going to show a varying oil temp. Head temp will change faster, but expect your oil temp to swing between 180+ and close to 201 under "normal" conditions. Climb a hill it goes up. Go down the other side, it goes down. All in "A day in the life" of aircooled driving.