While all the talk of jet blast is interesting, I doubt it is why the towed in my plane at SFO. Tight turning radius dictates using a mechanical means to shorten the turn radius (i.e. using a tug). Think of it like this, no matter how far you crank your car's wheels you still move forward in a radius when turning. Now if you got a jack under your front wheels, you could pull the font end in a perpindicular direction, turning the car in a radius based upon the rear wheels.
Likely, at that gate it is too difficult for the jet to make the radius required to clear the jetway and finish in the posiiton required for passenger loading. Therefore, they use the tug to turn the jet and bring it in.

Glenn