What is Facebook's supposedly compelling justification for for being behind Libra? On its face it makes about as much sense as having Facebook provide health insurance or a grocery store.
If successful, FB can reap billions in transaction fees to allow users to transact on their platform, be it for money transfer or for ecommerce.
I would not be surprised if Zuckerberg sees a future where the reserve currency is a digital one instead of the USDollar, where global currencies are valued in relation to Libra, rather than the $
Amazon does both of those. (some) Modern tech companies want to be everything platforms.
TL;DR Banking the unbanked with cheap micro transaction makes them milkable.