In the novel, albus dumbledore knows Voldemort's plan. In order to stop Voldemort, he sent Dobby as Voldemort's assistant to get more information and help. Dobby provided important information and help to Harry Potter many times in his novels, and finally sacrificed himself in the battle, winning valuable time and opportunities for Harry Potter and albus dumbledore.
Therefore, at the end of the novel, Harry Potter mentioned that Dobby was sent by Albus' younger brother, expressing gratitude and respect to albus dumbledore and Dobby.