According to PC Mag - "MailChimp has announced that users can now build ads on Facebook and its subsidiary, Instagram." Personally, I have no problem doing this within Facebook, but I know some people get intimidated by that ad manager and might be more comfortable working through MailChimp.
The big possible advantage is that "It also lets them combine MailChimp lists with Facebook-selected audience segments." If by this they mean that you can easily create custom audiences from your MailChimp mailing list, that could be a really cool feature. In fact, given many people's preference NOT to receive any email marketing / newsletters, this could be the key to MailChimp's future.