You asked two different questions. I'm not going to answer the one about why Facebook is creating a pegged cryptocurrency (other commenters have done a good job of that).

As for the value of blockchain here, there is absolutely none. Facebook is creating a currency and labeling it a cryptocurrency which denotes the use of a blockchain, when in reality considering they're the central issuing authority of the currency and are the ones who determine it's value/inflation rate (i.e. pegging as you rightly described). Therefore using a blockchain for decentralized settlement and provenance here serves absolutely no real purpose and you are correct in pointing out that they can just keep everything in a database (which is likely what they will actually do).