* To rescue Sirius Black: Harry believes Sirius is being held captive in the Department of Mysteries. He receives a vision of Sirius being tortured, and his friends Ron and Hermione, along with Luna and Neville, believe him.
* To uncover the truth about Voldemort's return: Harry believes the Ministry is deliberately ignoring or suppressing the truth about Voldemort's return, choosing to remain in denial. He wants to find evidence to prove the truth and expose the Ministry's deception.
* To confront Voldemort: Harry believes Voldemort has a powerful artifact hidden within the Department of Mysteries. He anticipates a confrontation with Voldemort, hoping to either reclaim the artifact or prevent Voldemort from using it to his advantage.
Ultimately, the trip to the Ministry is a pivotal event in Harry's life. It confirms his fears about Voldemort's return, forces him to face the dangers of the magical world head-on, and leads to a dramatic confrontation with Voldemort and his followers.