As I understand brevets, they were used 2 ways: 1) as an award, like a commendation and 2) to temporarily fill vacant ranks. If the latter case with your ancestor, it would be helpful to know when and where he was brevetted. For instance, if it came in the middle of the March to the Sea, supply being what it was, it might have been difficult to procure much of an officer's kit. It may have only been for a brief period of time, "mission specific," say a few weeks the company was without officers, or operating detatched from the regiment. At the end of the term of service, the brevetted officer returns to the last commissioned or appointed rank. In his case, sergeant was the last rank to which he was legally promoted. I have a hunch that they liked to see an officer in charge of soldiers rather than a non-commissioned officer, and would appoint one in situations where today an NCO would be considered more than sufficient.
Pine River Boys, Co I, 7th Wisconsin
"We're... Christians, what read the Bible and foller what it says about lovin' your enemies and carin' for them what despitefully use you -- that is, after you've downed 'em good and hard."
-Si Klegg and His Pard Shorty