Developer Javier Santos posted the screenshots of what he saw in the beta version, which is basically a hidden setting that would connect the two apps together. It is not something that can be easily seen and needs some terminal hacking to actually be able to see. It is also unchecked by default, so no need to worry yet that you’re secretly giving permission to Facebook. And since it is still in beta build, that is no guarantee that it will actually make its way to the final one.
As to how getting access to your WhatsApp account would enhance your Facebook experience, that is still unknown at this time. It may have something to do with your WhatsApp contact list so that they might appear on your Facebook suggested friends list. It may also have something to do with your WhatsApp conversations, where Facebook Groups or Pages will be suggested to you, based on the content of your messages. That last one sounds a bit iffy, so hopefully that is not the case.
Meanwhile, a new Documents section may also be added in the WhatsApp app so that the files you receive from your contacts are aggregated in just one place. Let’s see if these changes will be reflected in the next major update to the app.