The true answer is that Nixon needed no pardoning. He resigned before any formal charges could be brought against him. So the pardon was completely unnecessary, therefore a completely political move.

 He did it to end the watergate controversy, as he reasoned anyhow.  The decision cost him the election.  I guess the pardon itself became the news instead of Nixon.  It's very difficult to say if it was a deal from the day Nixon selected him for vp.  Nixon was certainly interested in saving himself.  Not having much to go on except Ford's record and reputation, I would have to say that Ford gave the pardon for the sake of the country.