I get the VC appeal of blockchain, but Facebook already has the money to fund it. Outside tech circles I don't think slapping a blockchain label on a product will make it more appealing.
I kind of hope the word "blockchain" should soon make it less appealing especially in tech circles.
It's a term without an accepted definition. A lot of useless branding is expected. Just you wait until IBM gets their hands on .. oh, they already did.
Both the Bitcoin-like zero trust ledger and the m-of-n trusted notaries model have useful use cases. While more descriptive terms would have been great, arguing definitions only goes so far.
The reason for the blockchain: Libra was founded by four bitcoiners (Morgan Beller, David Marcus, Kevin Weil, Christian Catalini) who wanted something like bitcoin, but not volatile, and run by sensible people, i.e. them.
As to blockchains, that's because this is the hype of the last few years, is it not?
As to currencies, IMO the vision of the people hyping bitcoin was that your pizza place would accept BTC (and not after converting their USD price to BTC) because their supplies would be happy to do as well. A single currency for the world would be interesting, but as economists have pointed out, devaluing your currency is a way to escape economic crises, but e.g. Euro countries can't do this any more, hence the problems the PIIGS countries had.