Answer:
He needs Brutus to support his plot against Caesar.
Step-by-step explanation:
In this scene, Cassius flatters Brutus because he wants him to join the conspirators. Cassius is planning to assassinate Caesar, and in order to do so, he wants help from all the "great men" of Rome, one of them being Brutus. Although Brutus eventually agrees, he will come to regret his decision later on in the play.