Oh and another one I use all the time is as an "identity transform" when selecting between filters, e.g.
dostuff () {
if [[ $1 = clean ]]; then
grep -v dirt
else
cat
fi | do_other_stuff
}
See also https://www.in-ulm.de/~mascheck/various/uuoc/ (via https://lobste.rs/s/rtvp2u/useful_uses_cat#c_0xpqkr )