Well,the Jiralhanae(Brutes) might have been for special assault missions on colony planets they thought had no resistance so the Jiralhanae could easily crush them.that makes perfect sense because Sangheili (Elites) were usually found around large-scale colony worlds with heavy military resistance at the time before the Great Schism(Covenant Civil War).The destruction of 04 might have sparked the San' Shyuum(Prophets) into believing the Sangheili couldn't stop one human so let the Jiralhanaes do some of the frontline fighting.It wasn't until the Changing of the Guard that they completely had control of the Covenant Military.