Depends on who you’re asking, but for Zuckerberg it’s mainly a way to monitor exactly how much is being spent by the user on every single transaction and then “monetise” it in some way (e.g. as a proof of an ad’s effectiveness, as someone described above). He also likes the idea of FB having a banking division that’s powerful but doesn’t have to deal with those pesky banking regulations.
As far as end-users go, it’s just another Venmo or Paypal clone, except with a lot more merchants accepting it in lieu of cash. It’s not good for criminal purposes because it isn’t designed with the user’s privacy in mind – quite the opposite. Zuck will happily give you up to the Feds in a second if they ask.
Some in the American financial services industry initially saw this as a quick way to bootstrap a competitor to China’s cryptocurrency effort (which could endanger the USD’s reserve currency status), but quickly realised what a disaster it would be (see this hearing) and bailed before they got dragged into it.
