This event will be the official unveiling of the entire games line up until E3.
They're having the event because they need a place to demo their new games to the press. It could also have one or two new announcements that they have decided would be better served at their own event or that they didn't want to cram into their E3 press conference due to having other stuff to show then. We'll see soon enough but I wouldn't be expecting megatons.