After the Animal Farm is established, Boxer adopts "I will work harder!" as his "personal motto." After Napoleon takes over, drives out Snowball, and sets himself up as dictator, Boxer adds "Napoleon is always right," to his motto of "I will work harder."
Both mottos show that Boxer, faithful and good-hearted, is a true believer in the cause of Animalism. At the point that Boxer adopts "I will work harder" as his motto, Animalism is still working as an ideal that benefits everyone. Boxer's motto inspires him, and that inspiration motivates the other animals to work harder too and to believe that the seemingly impossible can be accomplished. Boxer, in this context, is a good role model, showing the other animals that they can work together to accomplish goals.
After Napoleon takes over, however, and corrupts Animalism to benefit the pigs alone, Boxer's motto that Napoleon is "always right" becomes a hindrance. The other animals look up to and respect Boxer, so when he throws his weight behind Napoleon, this tyrannous pig becomes much harder to stop in his quest for power.
Boxer represents the Communist who was a true believer in the ideals of the cause, helping others to work together for the common good. Such as person was an inspiration until the system became corrupt. After that, the true believers' blindness to what was going on and unthinking support of tyranny—such as in the case of Stalin—became a problem that helped perpetuate evil.