Boxer was by far the hardest-working of the beasts of Animal Farm, and the farm would not have survived without his efforts. Without his presence, the humans would have probably won the Battle of the Cowshed.
... the most terrifying spectacle of all was Boxer, rearing up on his hind legs and striking out with his great iron-shod hoofs like a stallion... At the sight, several men dropped their sticks and tried to run. Panic overtook them...
Without Boxer's hard work and his persistence at working longer hours than any of the other animals, the windmill would never have been completed. His blind adherence to Napoleon's rules inspired the other animals. Like his presence at the Battle of the Cowshed, Boxer was essential to the animals' victory at the Battle of the Windmill. After the humans dynamited the windmill,
Three of them had their heads broken by blows from Boxer's hoofs.
But Boxer never got to enjoy his promised retirement, instead being sold to the horse slaughterer to be made into glue.