I think it was supposed to provide background to the character Ryan McCarthy. He's so into fighting and being the best, as Max mentioned to Jake he said that Ryan's dad was psycho and had him in Karate since before he could crawl.
It shows why Ryan is so aggresive, so cocky, so focussed on winning. He takes out his frustrations that his father causes him to have on the people that he pounds on and humiliates in fights: Jake being the main one in the movie.
So basically Mr. McCarthy is a microcasm to Jake's training in MMA.