The one major advantage is preserving sessions and re-attaching to them later, e.g. when you're ssh'ed into a remote machine or VM and you don't want to lose your setup.

Ratpoison could possibly be better (I keep meaning to give it a shot), although one thing I really enjoy about using tmux is being able to quickly open, shuffle, reorganize, and close panes with only the overhead of starting a new shell.

I happen to be a vim user. tmux, vim, vim-tmux-navigator, and some comfortable configurations in .vimrc and .tmux.conf can be a force multiplier for one's workflow. If you like vim, then it makes a lot of sense. If you don't, there's still probably some merit, but, maybe less of an incentive.

I've never used ratpoison, but just read into it. Sounds pretty great, actually. If the key bindings are simple enough, I suppose it could be pretty similar. But, as a sibling comment pointed out, it works most everywhere. Where I go, my configuration comes with me.

pro: use anywhere, including while ssh'd, and share sessions with others. con: not that easy to use -- kind of like vim or emacs.

I personally use tmux because it's "one window" and it is supported on every environment I can think of.