it pulls out windows into terminal tabs (in the UI), and handles panes like they were native iTerm panes (so same keybindings etc). Makes for a native-feeling experience while still getting the benefits of tmux.
iTerm2 will use tmux's scripting mode to give you a "tmux without tmux" kind of experience. Scrollback will work normally, and so will opening new terminal tabs and new windows.
It doesn't give you anything if you're already comfortable with tmux. But if you want your experience to "feel" more native, it's an awesome tool.