For me it's having two different terminals connected to same tmux session. I use tiling window manager and work on two monitors. Sometimes there is some stuff that I'm working on on my main monitor but I want to have quick look at the terminal. Because I have two terminals opened on both monitors I can just switch to the other one, press ctrl+f(my prefix)+number and have exactly same window as in my main terminal.

For anyone wondering you can achieve above by typing tmux new-session -t and giving it number/name of your current session.

Also there are minor things that are nice. Like if you need to open another terminal instance in the same directory as you're currently in.