Banks transfers take longer, banks transfers cost money and international banks transfers outside of SEPA network are a nightmare and they can get blocked/stalled for odd reasons.

Blockhain is a technical convenience - the Libra Association consists of 28 members. Everybody is more comfortable with money being stored on the public ledger rather than on a database server owned by Facebook. Blockchain is useful in cases where a collaborative/adversarial relationship exists between multiple parties. In this case this is regulators, governments, Libra association members and users. Blockchain keeps everybody honest, compliant and ensures data integrity.

It also removes some of legal responsibility from Facebook because they don't actually process the transactions themselves, it's peer 2 peer.

It's just marketing. Of course if you really want to you can bend a blockchain for this purpose, eroding it as much as possible, with the central validators and what not. A rose by any other name...

A transatlantic bank transfer will cost you over 30 euros in correspondent bank fees, in addition to the fees your bank and their bank charge. And you'll get no receipt for that, because banks suck.