Short version: iTerm with tmux control mode (tmux -CC flag) is still iTerm, only now with tmux benefits like persistent shell sessions. iTerm translates regular commands like new window and split pane into tmux commands and acts otherwise like regular iTerm. Mac might crash or SSH get disconnected and tmux stays running to save you from losing anything (well, if it's running on a server..).
In addition you tmux gives you shell sharing and allows using different devices alltogether. I sometimes use iPad or iPhone with Blink.app to connect to my always running sessions. You can connect to existing tmux session attaching with CC mode and iTerm will open windows and panes to accommodate.
One caveat if you require Mosh for network issues: it and tmux control mode aren't compatible. Also, I'd like to open tmux windows as tabs instead of separate iTerm windows, but that's a minor thing.
This page on iTerm's wiki explains things in detail: https://gitlab.com/gnachman/iterm2/wikis/TmuxIntegration
