For sure there's an element of promoting more cross-border trade, but all BRICS nations are commodity producers, China and to an extent, India do produce finished goods, maybe the capital flowing between others does expand the diversity of the economies to trade in more goods, who knows?

I'm not yet convinced of retail use. Stablecoins appeal isn't only the convience, it's the ability to get dollars/dollar exposure without a bank account and without limits on amount, fees, and clearing times.

In South Africa for example there are 2 private companies with local stablecoins no one wants them, people who are banked already have digitally interopetable money with merchants, and unbanked people have cash, so where do stablecoins come in?

Stablecoins have no local interoperability, the stablecoin appeal is to get money out of the local system in a relatively stable store of value. Dollars have performed very well as a savings tool versus holding cash, interest accounts, or even local equities.

What is the appeal for me to get a ruble, rupee, or renminbi stablecoin? It's not giving me any relative protection as a retail user.