Imagine what would be if Rocky movies were made with bad sound effects. I bet it wouldn't become a cult movie. It would be less beliveable than it is with great sound effects. And, the whole movie is about punching, so, those punches need to be perfect. 

You probably know that they are not actually punching eachother while recording the movie, and, you might have asked, how do they get characteristic sounds you hear. Here is something about it. 

For a movie "Out of the furnace" the director wanted to make Casey Affleck's punches sound realistic, so they hired a martial artist to create those sounds for them. They have recorded few sounds from punching human flesh, but, they also used many requisits that might seem weird to you. They  used dry pasta shells to create sounds of breaking bones, but, the artist also punches turkey meat, beef with a wet towel around it and many other things like watermelon. 

Another movie, "Grudge Match" with Sylvester Stalone and Robert de Niro, they have recorded real boxing punches from some boxing training, but, for some crucial moments, for some crucial punches with slow motion, they have used drum kicks sounds, howitzer cannon and prison door audio. Great, isn't it?

Although, there is computer science development and many movies use special effects for visual effects, but, audio effects are now, more than ever realistic and, today, if you hear the fist punching someone's face, it is most probably the real sound of it. 

Early filmmakers said that real punch sounds are not good enough for movies, because they don't contribute to the atmosphere, and, because of that, they used many different tools in order to create sounds that will contribute to the excitement. So, in many movies you watched, you might have heard billiard balls, towels punching the wall and similar things. Those sounds are often called "Hollywood punches". 

Another thing, Bruce Lee didn't actually beat the hell out of people to make those sounds you probably remember. For those kung fu movies, they used bamboo sticks to punch the board and those were the sounds you heard. 

In 1980s, directors found out that they don't need to use sounds from their sound libraries, they found out that they can create sounds for every punch in a scene, because then they got higher budgets. Then, the era of violence over beef, turkey meat and other objects has started and that is the reason why we enjoy in movies more than we could enjoy before - movies of today have better sounds.