Although she has a minor role in terms of action, Bramimonde's commentary is crucial to the poem's larger philosophy: she illustrates the uselessness of non-Christian religion and the fundamental weakness of the Saracens' military power.
It's almost if the Franks planted her in Spain to encourage dissension and sow despair. When she sees Marsile with his stump hand, she immediately turns against the gods that failed to protect them and starts to make the connection that something supernatural is protecting the Franks. If only she could see those angels.
Other Saragossans are also peeved about their gods, but Bramimonde is the only one to speak up to the Emir against further war. When his messengers tell her he's planning on attacking Charlemagne, she cackles sarcastically:
"What rubbish I hear!
Those gods of ours have given up the fight.
At Roncevaux they did us a colossal bad turn,
They allowed our knights to get killed." (195.2714-17)
She is the only pagan who realizes on her own that Christianity is superior to the gods she used to worship and that further war is useless. This makes her a model citizen of a Franks-conquered land: someone who recognizes that the conquerors' military superiority derives from their true religion—and then surrenders. Understandably, this makes her unpopular with the other Saracens who are still committed to war.
Both her husband and the Emir's messengers tell her to shut up, silencing the voice of internal criticism. Additionally, she has two X chromosomes, so that also means she shouldn't be voicing an opinion. Women are to be silent and decorative in this world.
Her dissent also makes it credible that of all the pagans force-converted to Christianity, she is the only one who is baptized "out of sheer conviction." Charlemagne reports that after hearing many sermons and Christian stories, she actually "asked to become a convert to Christianity" (290.3987, 3980). Taking a new Christian name symbolizes her complete transformation to a Christian woman living in a conquered land.