This one goes hand in hand with "Ambition." Before the heroes of Back to the Future can achieve their goals, they have to suffer some pretty oppressive defeats, and we get a front row seat to see if they're able to bounce back.
It would have been great for George, for example, if the knocking-Marty-out scheme had panned out, but what was really character building was his unexpected encounter with Biff instead: he suffers a (temporary) defeat before collecting previously unknown strength and delivering a wallop to his nemesis that would have garnered Chuck Norris' approval.
Questions About Defeat
Something changes inside George to turn him into a successful writer in 1985. What was it, and what did Marty have to do with it?
Some people wallow in defeat, while others use it to motivate them. What type of person is Marty?
If Marty hadn't gotten back to the future toward the end of the film, that would have been a pretty brutal defeat. How do you think he would have handled that one?
What role did his defeat at the hands (literally) of George play in Biff's personality adjustment?
Chew on This
If Marty hadn't experienced defeat in his own life, he never would have been able to empathize with his father's plight.
It is possible to suffer a defeat without become defeated.