Why do the conspirators need Brutus to be a part of their plan ?

Brutus is a noble, well-respected senator in Rome.  Even though Brutus sided against Caesar when Caesar fought with Pompey, Caesar gave Brutus a seat in the Senate.  Caesar did this for two reasons: he knew Brutus could be very dangerous, as he was a well-liked Senator and had the support of the plebians, and because Caesar wanted to keep Brutus close to him, as Brutus was a powerful man.

Cassius knows that Brutus is well liked, so if Brutus supports his plan, so will the plebians who support Brutus.  Cassius also knows that Brutus is close to Caesar.  Shakespeare portrays Caesar as trusting of Brutus, which Cassius hopes he can use to the conspirators advantage.  While Caesar may have been somewhat trusting of Brutus, the real historical Caesar was probably still wary of him.