When a football team uses a really big guy as a back, do they expect him to do really good?

Like when the Bears used William Perry, the 49ers used Guy McIntyre, when the Lions used Shaun Rogers, and when the Packers use B.J. Raji, does that team put them in because they think that it will actually work better than their starting fullback, or just for novelty purposes? I find it hard to imagine that they would do it just for fan reaction, as they kind of would want to win.