I think a fullback has just as much of a specialty role as a third tight end, but a good one brings more versatility to his role. He's able to lead block effectively with a shorter build that allows him to root out a linebacker. He can catch the ball and he can run it. Obviously, it is not an every-down position in today's NFL, but it still has a place, especially if the player can play special teams. On game days, he would take the place of the third running back on the active roster.
I don't know if Ralston is the player to do that, but I'm intrigued by him. McCarthy has shown that he's willing to use a fullback if he has a good one.