Don Black and Walter Scharf wrote this for the 1972 movie of the same name. It was the sequel to a movie called Willard, which was remade in 2003 starring Crispin Glover.
In Willard, Ben is a pet rat who is trained by a human named Willard that has learned how to make rats to his bidding. Willard has the rats kill for him, but at the end of the movie they turn against him.
In the sequel, Ben, a young boy befriends the rat, but the rat turns evil and recruits other rats to attack humans. In the remake of Willard, Ben becomes an enormous super-rat.
Despite being about a rat, the lyrics are touching and sentimental, and make no reference to rodents. This made it a sturdy song about friendship:
They don’t see you as I do I wish they would try to
I’m sure they’d think again if they had a friend like Ben
Many people were very moved by the song, which was marketed independently of the movie, as the true meaning would have irked some listeners.