[Nacraile]

1. Outer tmux on local/primary machine, for your usual daily use-case (i.e. managing multiple terminal sessions).

2. In outer tmux, ssh to some other machine you need to administrate.

3. screen because you need multiple terminal sessions on that machine, or need detachability/reconnectability for some long-running process.

[jethro_tell]

Also, many older production machines don't have tmux, so nesting a screen session happens. This is starting to not be a thing, but it was a thing for the trailing 10 years.