PayPal is not available in a lot of countries (Turkey would be one example). Western Union might work, but the fees are high and there's much more friction. In case you're based in the United States: Would you use Western Union over Venmo / Square Cash. If you're based in Europe: Would you use Western Union over SEPA? Libra has the potential to provide a low friction environment (like Venmo) supported by local companies that's available globally.

And while Libra definitely isn't perfect I think the set of trade-offs they chose are pretty reasonable. Another advantage over Venmo, Western Union, etc. is that the Libra will be run by a multiple independent companies, instead of locking you into a single company which is the case with the currently existing solutions (with the exception of SEPA).