Imagine you want to diff outputs of two scripts which operate on the same resource (file). Compare contents of the file before and after a transformation in a single pass, for example.
With process substitution, like diff <(cat myfile) <(reformat myfile; cat myfile) both substitutions are started in separate subshells at the same time, so you would need a semaphore, to ensure the second process substitution is started after after the first one - otherwise you might hit a race condition.
One option is to use a named pipe for that, but it involves the creation (and cleanup) of the pipe device file.
Another option is to use a co-process (Bash 4+) for that. cat command running as the co-process would basically act as an anonymous pipe!
echo BEFORE >data.txt
(
coproc cat
diff -du \
<(sleep 5; cat data.txt; echo >&"${COPROC[1]}") \
<(read <&"${COPROC[0]}"; echo AFTER >data.txt; cat data.txt)
)
(sleep is for demonstration purposes only)
will output
--- /dev/fd/62 2018-11-15 21:41:17.771271807 +0000
+++ /dev/fd/61 2018-11-15 21:41:17.771271807 +0000
@@ -1 +1 @@
-BEFORE
+AFTER