On paper, you're basically entirely right. Stablecoins including FB are all smokescreens to sell managing fiat on an internal database for you with "blockchain" wrapped around it. This project and stablecoins in general have nothing to do with cryptocurrencies. With that said i can tell you at least one solid reason why any issuer would do it. At a bare minimum they can earn overnight on the holdings since stablecoins don't kick back interest to date. Although with stablecoin competition that should probably start happening until it squeezes to 0 and then it really wouldn't make any sense except to like monetize spying on you I suppose or something. But to take an example, at this point tether is generating 75mm a year for free for it owners at 0 risk. It's the crypto equivalent of a "free" checking account that doesn't pay interest. Not that any exchange was paying interest for fiat deposits in the first place so i'm not sure to what extent that's relevant for cryptoexchangess, but for FB obviously it would since they're starting from 0.
With that said I can imagine one reason why users would care for USD stablecoins. The US has made owning and dealing with USD directly an absolute horrendous nightmare for international users and US citizens abroad following FATCA et al. And this way you can claim you never owned or touched USD which means whoever is holding stablecoins for you instead of USD doesn't have to file truckloads of paperwork for you to open the account. That's a regulatory loophole though one could imagine the US would start complaining about if this really took off.
There's one remote reason I can think of that people would flip to stablecoins vs fiat if they had some tax view that that wasn't a constructive sale and it could benefit from some likekind kind of treatment since its a "crypto" and not a "fiat". It's obviously a silly argument and there's no way the IRS would fall for it (nor do they even accept it between regular cryptos) but i can't speak for other jurisdictions.