Brownie points in modern usage are an imaginary social currency, which can be acquired by doing good deeds or earning favor in the eyes of another, often one's spouse.
You can trade these brownie points with other similar help needed.
How much is a point worth? This can be based on the rating of the help that you did for a person.
What comes to mind is also Black Mirror episode "NoseDive", where people are rated from 1-5.
Another thing that I think was the motivation is the Hippie culture, which had a rejected the mores of American life, and were alienated from the middle-class - I believe they basically survived by helping each other.
The gray economy (or informal economy) is often an unregulated system. It is centralized in the sense that we use fiat currency. Brownie points can be earned without the need for fiat. I imagined a world where we need to transact with someone whose reputation we do not know about. Usually you could check how many friends they have on a social network - but have they been good friends?
Brownie points would help settle that.
But Good people generally don't want to be noticed for their good deeds, and you don't need points for people to help you.
I agree that mostly, good people don't brag about their good deeds because it looks cheap. For this use-case, a private person can prefer to not to show publicly how many brownie points they've earned.