Final answer:
The conspirators needed Brutus to join their plan because of his honorable reputation, which they believed would lend credibility and a sense of legitimacy to their cause against Julius Caesar.
Step-by-step explanation:
The conspirators needed Brutus to join their plan primarily because of his honorable reputation. Brutus was well-regarded in Roman society as a man of integrity and principle, respected for his dedication to Roman values. This made him a powerful figure whose involvement in the conspiracy could lend credibility and moral weight to their cause against Julius Caesar. His association with the plot was believed to make it appear less like a mere power grab and more like a noble effort to save the Republic.
While other factors such as political influence and financial resources could have been beneficial to the conspirators, it was Brutus's reputation of honor that was deemed indispensable. With Brutus on their side, the conspirators hoped that the Roman public and the elite would be more likely to see the assassination of Caesar as a legitimate act to prevent tyranny and preserve the Republic rather than an illegitimate mutiny.