It's simple: Zuck wants to do basic stuff on FB like have an easy link that lets people split a bar bill or have an easy link for buying into a bridal registry, and his lawyers keep telling him stuff like "don't do that or FB will be regulated as a bank" or "don't do that or FB will need to do KYC" and he's sick of hearing that.

Libra's sole purpose is to let FB do this kind of stuff while sidestepping the regulatory red tape.