The revelation of Portia's death has two main effects: it explains Brutus' behavior, and it helps to reconcile Brutus and Cassius.
In Act IV Scene 3, before we learn of Portia's death, we see Brutus acting in a puzzling and uncharacteristic way. Given that he is engaged in a full-scale war, it is madness of him to go out of his way to attack, belittle, and bait Cassius, his co-commander. Brutus' words go far beyond what might be expected from his picky sense of morality. He seems almost to delight in provoking Cassius, as if his object were to cause a breach with him:
By the gods,
You shall digest the venom of your spleen,
Though it do split you, for, from this day forth,
I'll use you for my mirth, yea, for my laughter,
When you are waspish.
Cassius repeatedly attempts to deflect his insulting remarks, but Brutus is in no mood to listen until Cassius is reduced to a miserable state:
Come, Antony, and young Octavius, come,
Revenge yourselves alone on Cassius,
For Cassius is aweary of the world:
Hated by one he loves; braved by his brother;
Check'd like a bondman; all his faults observed,
Set in a notebook, learn'd and conn'd by rote,
To cast into my teeth.
Brutus' mood then suddenly turns, and after the intervention of the poet, which provides a touch of low comedy, he confesses to Cassius that it is the news of Portia's death that has made him so ill-tempered. Cassius, of course, is horrified and saddened by the news:
How's caped killing when I cross'd you so?
O insupportable and touching loss!
In this sense, Portia's death draws a line under the quarrel between the two and allows it a credible conclusion. Nevertheless, the details uncovered during the quarrel help the audience to understand why Brutus and Cassius are not more successful in their military struggle against Antony and Octavian.