Boxer's memory is glorified for future generations and Napoleon is able to garner a financial profit from selling what's left of Boxer. The other animals' memories of what really happened will fade, and Boxer's dedicated work ethic will be held up as a goal for future animals to emulate.
It is also a significant event in light of Old Major's speech before the revolution. In that speech, Old Major predicted that Jones would, once he lost his strength, "sell you to the knacker, who will cut your throat and boil you down for the fox-hounds." This is the horror that all animals must face under human rule. So when Boxer is sent off to the horse slaughterer by Napoleon, for the reasons mentioned in the previous response, it demonstrates that the events of the revolution have come full circle.
Napoleon benefits financially, getting whatever Boxer’s carcass fetches at the knackers. He also does not have to feed a worthless animal who’s unable to work. He gets to use the land that had been set aside for the retired animals to grow crops for his whiskey. He is hypocritical because he took advantage of animalism and used it to his own advantage.
Boxer loses his strength and intends to retire. Without the ability to work for Napoleon, Boxer loses his value in Napoleon's eyes.
There was a promise made and a pasture set aside to be dedicated to retired animals and Boxer believed he would have a chance to spend his last days in leisure there. Napoleon betrays Boxer by denying him the right to retire and tricks him into believing that he is going to be treated for the injury he has sustained.
Not only does Napoleon lie but he also betrays the promise which Boxer had been working toward, the right to which he fully deserved.