If you're a Linux user, a logical extension of this idea is to use a tiling window manager, and then you get the paneled window management for everything, not just your terminal.
Being able to detach and re-attach to sessions is still really useful, but I never find myself wanting to use tmux for splitting up a terminal window when I'm already using a tiling window manager.
I never got into those, because I have two modes of working. When I'm writing code, my hands are on the keyboard and I have several splits open and a vim session. When I'm doing anything else, I like to use a mouse because it gives me random access to windows.
"I never find myself wanting to use tmux for splitting up a terminal window when I'm already using a tiling window manager"
I don't want to launch extra terminal instances every time I want to split my terminal window. Also, there's no easy way to detach or reattach to separate terminals as there is to tmux. Nor is there an easy way to programmatically pass information to or from them while they are in the background or off-screen, etc.
That's not to say that separate terminal windows aren't useful. They are, and I sometimes use them myself (particularly for drop-down terminals like guake or terminals in i3's scratchpad).
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.
Being able to detach and re-attach to sessions is still really useful, but I never find myself wanting to use tmux for splitting up a terminal window when I'm already using a tiling window manager.