In this important scene we see Brutus wrestling with himself and deciding to join the conspiracy. His insomnia ironically contrasts with the sleep that Cassius has had with no qualms of conscience - it appears than in this play it is the innocent that are troubled with sleeplessness. This scene also sees Brutus assuming the lead of the group of conspirators and replacing Cassius by taking away the words of an oath and filling their place with images of heroic Romans.
Brutus then goes on to show his leadership - despite the beliefs of the other conspirators regarding the usefulness of Cicero or the danger of Marc Antony, Brutus has his way and overrules any objections - something that will come back to haunt him and the conspriators later. It is interesting that Cassius, who is clearly shown to be more canny and worldy-wise than Brutus, champions decisions that would have ensured their success, yet he gives in to Brutus easily.
Lastly, the relationship between Portia and Brutus shows what a strong character Portia is. Her desire to know the plans of her husband, and her proof that she can take the knowledge by wounding herself in the thigh, makes Brutus feel he is undeserving of such a wife. It is only because Ligarius arrives that he is prevented from telling Portia all. Ligarius incidentally also reflects the esteem with which Brutus is held - despite his illness he goes with Brutus at his bidding.