Like with Dragon Ball Z, it shows you the reality that the hero doesn't necessarily have to win against the evil doer just because he is the good guy
Dragon Ball Z shows the true reality that Goku despite being the hero might have just defeated the bad guy due to just dumb luck, for instance Goku was just lucky that Yajirobi managed to cut off Vegeta's tail after he transformed into a Great Ape
Also since Frieza was stronger that Goku in the first part of their fight, Goku was lucky that Frieza killed one of his friends first so that he could get angry enough to unlock his Super Saiyan transformation
Don't you think this all reflects on how things go with good vs evil in real life?