I think that the primary motivation behind the filming of Steinbeck's novel is to inspire.  The filmmakers have taken Steinbeck's social realism and really moved it to the idea that there is an almost mythological quotient to the construction of Tom Joad.  Consider the epic- like ending where Tom speaks to Ma about his need to leave.  The manner in which this scene is filmed features Tom as almost a superhero, with the slightly blurred surroundings, almost to accentuate Tom's clarity in a world that lacks it.  The manner in which Ford takes the film is one where the social struggle, and the intensity with which the battle between those how have power and those who lack it is waged, is almost secondary to the idea that the Joads and others like them are the automatic heroes of the film.  The motivation here is to bring out the idea that those who might be considered the dispossessed are actually heroic and deserve the praise and respect of the audience.  This is in stark contrast to the historical reality in which these voices were silenced by those in the position of power.  It is evident that Ford wishes to use the film as possessing a level of mythologizing these individuals, vaulting them to a condition that was not seen in the historical condition of the time period.