I've been a long-time user of both i3wm and tmux. If I had to mention a single reason for using tmux, it would be sessions (ie restarting X11 would keep the session alive). There are also some features like searching for text in open windows, that aren't available to a tiling window manager. It's not only about tiling. I also found that restoring tmux sessions (tmux-resurrect+tmux-continuum) was faster than restoring i3 workspaces.