Imagine a legacy script. Very long, complex and business critical. No time for a rewrite and no fixed requirements. But you want to have timestamps added to the output it produces.
Search and Replace
One way to do this is find each and every echo and replace it.
echo "Doing X with $file."
becomes
log "Doing X with $file."
and you implement log() as a function prefixing the timestamp. The danger here is to not replace some echo that needs being redirected somewhere else.
Alternative: Wrap in a Subshell
Instead of modifying all echo's one could do the following and just "wrap" the whole script:
#!/bin/bash
$(
<the original script body>
) | stdbuf -i0 -o0 -e0 awk '{ print strftime("%Y-%m-%d %H:%M:%S"), $0; }'
You can drop the stdbuf invocation that unbuffers the pipe if you do not need 100% exact timestamps. Of course you can use this also with any slow running command on the shell no matter how complex, just add the pipe command.
The obvious advantage: you do not touch the legacy code and can be 100% sure the script is still working after adding the timestamps.