In the play Julius Caesar Brutus is essential to the success of the conspiracy for many reasons.
Although Cassius initiates the conspiracy and manipulates Brutus to join the conspiracy, Brutus is already concerned about Caesar in Act I. Brutus reveals that he is "with himself at war" about what to do about Caesar's ambition (I, i, 134). Brutus reveals that he is deeply attached to the Roman Republic. Part of the reason for this attachment is the role that Brutus' ancestors played in establishing the Republic. As Brutus debates what will result from the people's desire to crown Caesar and Caesar's ambition, he concedes that Caesar is like a "serpent's egg" and must be "killed him in the shell" (I, iv, 633-635).
Brutus's concerns are important because they reveal to the reader (and he will reveal to the Roman people at Caesar's funeral) that his reasons for joining the conspiracy are noble. He desires to preserve the Republic.
It is for these reasons why Cassius and Casca both deem it necessary that Brutus join the conspiracy. Casca compares Brutus' "countenance" to "alchemy" in Act I scene iii. This implies that Brutus is not only well-respected, but that he is influential on the Roman people. When Brutus becomes involved in the conspiracy, it legitimizes the cause. Cassius knows that his tumultuous relationship with Caesar may imply that Caesar was killed out of jealousy. But if Brutus leads the conspiracy, the people will support what the conspirators did because as Mark Antony declares in Act V after Brutus dies, "here lies the noblest Roman of them all/ All the conspirators save only he/ Did that they did in envy of great Caesar" (2753-55).