Stallone literally was a starving actor on the set of Rocky. They couldn't afford craft services. Or much of, well, anything.
Rocky was made like a student film, with actors doing their own driving, and the film being shot on 8mm film at a breakneck pace. Due to their low budget, they often had to improvise. To shoot the interior driving sequences, a cameraman held onto the front of the car for dear life.
By literally hitting the pavement—and a few sides of beef—the filmmakers ground Rocky in gritty reality, and the film's realism makes his success story all the more poignant.