what does it add over screen which i don't even need to install?
If screen crashes you lose the sessions, Tmux maintains state.
Screen does not have UTF8 support, tmux does.
Otherwise just a bunch of more sane original defaults in tmux to make things much familiar.
In 2026 if given a choice between screen and tmux to use/learn, most are going to go with tmux.
cmux? I don't know and it's not even a good comparison.
tmux? https://www.google.com/search?q=tmux+vs+screen