Besides persistence, terminal multiplexing is one of the greats things of tmux. If I'm programming on a remote machine I don't want to open a new ssh connection from each of the five terminals that I need. Seems tedious when I can just create a new window inside tmux. Even locally, I don't want to have 10 terminals opened (or tabs), tmux lets you have multiple terminal windows/panes from a single physical terminal and lets you easily group and switch between them into what they call sessions.