A good use case is to redirect output into a file you only can modify with sudo. Trying to do it like this doesn't work:

  sudo echo "fs.inotify.max_user_watches=1000000" > /etc/sysctl.conf

as the redirection happens within the shell, thus without root permissions. However, the following does work:

  echo "fs.inotify.max_user_watches=1000000" | sudo tee /etc/sysctl.conf

as the writing is done by the elevated tee process.

That seems like an anti-unixism (operating with multiple EUIDs in a single process group), though I'm not too sure what the equivalent unixism would be—maybe something with a daemon and a spool directory? Or a user-writable FIFO, redirected to the correct file by the elevated user in a separate command?

I use tee to save log files from long running programs. The chatter goes to stdout, and informs me about progress. But the tee also saves the chatter, so I can review it if something goes wrong.

You could get a similar effect in other ways (e.g., tail -f, or terminal scrollback), but I find tee is convenient.

Often I'll do something like

    long_running_thing | tee long_running_thing_stdout

and want to see the first few lines of output before going to do something else. The pipe causes buffering on stdout, though, so I have to sit there until 4k of output has come out on stdout.

As seen on https://unix.stackexchange.com/questions/25372/turn-off-buff... , either `unbuffer long_running_thing` or `stdbuf -oL long_running_thing` causes it to output linewise instead of 4k at a time.